Solved Benefits of GPT vs MBR Primary and MBR Logical Partitions ?

[alan10]

I wish to hold some programs and some data on non-system partitions with no boot requirements.

What are the benefits (performance and otherwise) of using a partition :-
Logical on my MBR style HDD
Primary on my MBR style HDD
Primary on my GPT style HDD

NB
I have adequate unused space on both the HDD,
My system is on an OCZ SSD
My motherboard is not uEFI capable hence cannot boot GPT.

Regards
Alan

 

[TVeblen]

From the user's standpoint there really is no perceptible difference in partition type for user data and installed programs. In everyday use the F: drive is the F: drive and neither you or the system give a hoot about it's type.

The main benefits of GPT partitions are compliance with the requirements of UEFI systems, the recognition and use of partitions or hard drives larger than 2.2TB, and the ability to have up to 128 primary partitions.
If none of the partitions on a data hdd are larger than 2.2TB then you do not need GPT to use them.

The main benefit of a MBR primary partition is the ability to boot an OS from it.
Windows creates primary partitions first by default. The reason: "why not?".

The main benefit of a logical drive is that it allows a MBR disk to have more than 4 partitions, since a MBR disk is limited to only 4 primary partitions.
A logical drive does not perform any faster, slower, or differently than a primary partition.

Be careful when creating that 5th partition on a MBR disk. You do not want the system to create a Dynamic Disk. Make sure it is going to create a logical drive.

So you just decide what you have and what you need to do. Don't over-think it.
Hope that helps!

 

[alan10]

Thanks.

My MoBo can only boot MBR but can access both programs and data partitions on my GPT HDD.
My largest existing HDD is 1 GB.
Is it probable that my existing 4 year old MoBo and BIOS would fail to deal with a 3 GB partition or even a 3 GB HDD,
regardless of whether it was MBR or GPT style ?

Supplementary Question on (Extended) Partition Tables :-
Are all such tables read from all HDDs upon start-up and the Partition Letters and MFT/Directory etc. locations copied to RAM,
or does the Operating System trawl through these tables on each HDD each time I try to access an item in partition F:\ ?

Regards
Alan

 

[TVeblen]

Just a clarification: your motherboard only looks to see if there are drives, and then if there is anything to boot on them.
It is capable of booting MBR, GPT, Linux, Sun, Mac OS, anything compatible with the system. The file format on the drive is of little concern to the BIOS.
It can deal with large drives.

It is the operating system that has the troubles. Windows 7 can startup from GPT and MBR disks. It just can't boot from a partition larger than 2.2GB. For anything larger than that it needs the GPT disk.

A pure UEFI system needs a GPT disk to boot. But a hybrid system can boot from both. And a legacy BIOS system can boot from both. It is important to understand that the BIOS/UEFI system is completely separate from the OS.

Supplementary Question:
Yes and no. When you start your computer, during the OS startup, the OS makes a list of the attached drives, the partitions, and the file table. Once you are on the desktop all of this data has been "recorded".
When you go to access these partitions the OS does, I guess, "trawl" through the tables to access the item you are looking for, but all of this is done so fast it is not worth mentioning. If you can see the file on your screen the work has already been done.

 

[alan10]

Many thanks - all my concerns now resolved.

Regards
Alan

 

[milindsmart]

MBR was always a de facto "standard"... A hack if you will. Born in the days of MS-DOS.

GPT is a real, well-designed standard. You should ALWAYS prefer GPT unless you have a reason not to. MBR is one of those ideas that should be (now) confined to history books.

 

[alan10]

A pure UEFI system needs a GPT disk to boot. But a hybrid system can boot from both. And a legacy BIOS system can boot from both. It is important to understand that the BIOS/UEFI system is completely separate from the OS.

Ever since I used DOS, long before Windows,
I understood that the O.S. was software that was installed on a Disk,
and that the BIOS was held in non-volatile storage device on the motherboard.

I assume that by"pure UEFI system needs a GPT disk to boot. But a hybrid system can boot from both"
you are referring to capability of motherboard plus its non-volatile storage device.

Exactly what is a legacy BIOS system ?
Do I need to make any changes to my "normal" motherboard and BIOS in order for it to become a legacy BIOS system ?
Are you saying that any Windows XP computer onwards can boot from GPT ?
I understood that was not possible for my hardware + BIOS to boot from a GPT disk.
Are you saying that I was misled ?
About 25 years ago I bought second hand a 1982 vintage Compaq Luggable Computer,
with 5.25" Floppy Disks and DOS 3.3? and an optional 20 MegaByte HDD
Are you saying that the motherboard+BIOS would have supported booting from a 4 TB GPT HDD ?

GPT is a real, well-designed standard. You should ALWAYS prefer GPT unless you have a reason not to. MBR is one of those ideas that should be (now) confined to history books.

The Twelve Times Table was well-designed, and as an infant it helped me with things like 9 * 7 = ?
I no longer get any benefit from the "Well-Designed Twelve Times Table"

Windows is a flawed design with race hazards built in.

Elegant "well-designed standard" Disk standards are worthless to me when Windows can ruin everything,
because Windows numbering of SATA connected Disks is based upon "order of enumeration",
and I once had both secondary Disks given identical Disk ID's due to a race hazard,
with the result that Windows cried FOUL and randomly chose to put one of the disks OFFLINE,
and that Disk which was almost full now has two RAW partitions but the big archives are now UNALLOCATED SPACE.
N.B. Most data recovery tools were useless.
Then I found freeware which did a perfect job :-
Lazesoft Recovery Suite Home Free Download


I am looking for BENEFITS from using GPT.
129+ Partitions and use of 17+ TB drives are to me absolutely worthless "Benefits" of GPT.
Several third party tools such as Partition Managers and Partition Imaging backups are far superior to anything built into Windows,
and the fact that some of these tools disable GPT capability on freeware versions is a PRICELESS benefit of MBR.

Regards
Alan

 

[milindsmart]

Ever since I used DOS, long before Windows,
I understood that the O.S. was software that was installed on a Disk,
and that the BIOS was held in non-volatile storage device on the motherboard.

I assume that by"pure UEFI system needs a GPT disk to boot. But a hybrid system can boot from both"
you are referring to capability of motherboard plus its non-volatile storage device.

No... "pure UEFI" system means firmware that only exposes the Extensible Firmware Interface to operating systems.

Also please don't confuse them : GPT is only a section in the UEFI specification... This does NOT mean that GPT disks can be booted in UEFI systems.. In fact, Windows is the ONLY operating system with this limitation... In Linux, for example, all four combinations of firmware and disk partitioning layout are supported.

Exactly what is a legacy BIOS system ?

A legacy BIOS system is one whose firmware behaves essentially the same as the IBM PC/AT model... Where the mechanism of interaction with an OS is primarily in the form of interrupts. It also initializes the processor in 16-bit real mode, which is why every x86 processor today STILL starts in real mode : it's a de facto standard, that's very restrictive, considering it was conceived 25 years ago.

Do I need to make any changes to my "normal" motherboard and BIOS in order for it to become a legacy BIOS system ?

It's kind of hard these days to say what's "normal" : Most old motherboards are BIOS, most new ones are UEFI-capable. Most motherboards are "hybrid" in the sense they can switched into one or the other mode through a setting in the firmware setup (BIOS screen).

Are you saying that any Windows XP computer onwards can boot from GPT ?

No, only Vista x64 SP1 and above can boot from UEFI, which Microsoft has decided will be the criteria for being able to boot from GPT.

About 25 years ago I bought second hand a 1982 vintage Compaq Luggable Computer,
with 5.25" Floppy Disks and DOS 3.3? and an optional 20 MegaByte HDD
Are you saying that the motherboard+BIOS would have supported booting from a 4 TB GPT HDD ?

Nope. The motherboard is too old to support anything other than Cylinder Head Sector addressable HDDs, let alone such large disks. GPT doesn't really matter, it's up to the operating system to support.

The Twelve Times Table was well-designed, and as an infant it helped me with things like 9 * 7 = ?
I no longer get any benefit from the "Well-Designed Twelve Times Table"

You don't need the twelve tables because you know how to multiply, and also you have calculators and computers. Disk partitioning schemes are like multiplication algorithms : everyone uses and depends on them.

Windows is a flawed design with race hazards built in.

Earlier. Not now.

Elegant "well-designed standard" Disk standards are worthless to me when Windows can ruin everything,
because Windows numbering of SATA connected Disks is based upon "order of enumeration",
and I once had both secondary Disks given identical Disk ID's due to a race hazard,
with the result that Windows cried FOUL and randomly chose to put one of the disks OFFLINE,
and that Disk which was almost full now has two RAW partitions but the big archives are now UNALLOCATED SPACE.

There are limits to technology, old or new. It tries to give unique IDs as far as possible. Anyway this could not have been Vista +. XP did suck in a few ways.

I am looking for BENEFITS from using GPT.
129+ Partitions and use of 17+ TB drives are to me absolutely worthless "Benefits" of GPT.

1. Robust and fault tolerant partition tables
2. Well known partition type for bootloaders
3. No primary-logical nonsense
4. No clashes in disk type IDs
5. Clashes are definitively solved with GUIDs
6. Can even contain other partition tables nested

Several third party tools such as Partition Managers and Partition Imaging backups are far superior to anything built into Windows,

Yes I agree... Windows disk management is for basics, not advanced operations. Paragon partition manager and Active@ Disk management have served me well.

and the fact that some of these tools disable GPT capability on freeware versions is a PRICELESS benefit of MBR.

Which? Not any more.

Storing data in an MBR partitioned HDD is like carrying knives in a thin polythene bag. GPT is a rucksack. Just because tools have managed to work around some limitations of MBR does NOT mean that it's now a good standard. I have messed a couple of hard drives because I created the 5th primary partition, because one was hidden. It's like 12 x 12 = 0.

 

[alan10]

Exactly what is a legacy BIOS system ?

A legacy BIOS system is one whose firmware behaves essentially the same as the IBM PC/AT model... Where the mechanism of interaction with an OS is primarily in the form of interrupts. It also initializes the processor in 16-bit real mode, which is why every x86 processor today STILL starts in real mode : it's a de facto standard, that's very restrictive, considering it was conceived 25 years ago.

Thanks for clarification.

There are limits to technology, old or new. It tries to give unique IDs as far as possible. Anyway this could not have been Vista +. XP did suck in a few ways.

Under XP I discovered that when CMD.EXE invoked REG.EXE to READ (not modify) a key
and piped both STDOUT and STDERR to the same log file,
for keys that had sub-keys that had no access there would be simultaneous answers on both output streams for the same interrogation,
and MOSTLY both streams were logged, but sometimes neither were logged, and at other times only one was logged but not the other.
XP was successful better than 99% of the time when my batch script and the output log file were in partition C:\
When I moved only the log file destination to a partition at the other end of the HDD,
the success rate fell below 90%.
The problem was perfectly cured by disabling the output cache
(I think it may have been called "Enable Safely Remove")
Perfect cure - but Windows started up 3 times slower.
You are right - XP sucks.

1. Robust and fault tolerant partition tables
2. Well known partition type for bootloaders
3. No primary-logical nonsense
4. No clashes in disk type IDs
5. Clashes are definitively solved with GUIDs
6. Can even contain other partition tables nested

I have heard previously that GPT partition tables are fault tolerant, but do not see how that is possible.

An MBR has only two tables, I think they are called Primary and Extended
Break the primary and you lose everything.
break the extended and you lose all the Logicals.
BUT you have only two such tables to be broken, and as of 9 months ago the Professional licensed version of Minitool Partition Wizard was NOT capable of REPAIRING a broken GPT Partition Table,
even though it could shrink and extend GPT partitions.

I understand that every GPT partition is preceded by its own 3 MB GPT Table and that one GPT table links to the next GPT Table,
which MIGHT be the complication that Minitool Repair could not cope with.

If you have 128 partitions :-
Using MBR you have only 2 tables that may be broken,
(and 4 or 5 years ago freeware Mintool version 4.? fixed my Laptop which had lost ALL 7 partitions.)

Using GPT you have 128 tables that may be broken, and that loses all the subsequent partitions,
unless of course there are "redundant" backward pointing links to traverse the chain in the reverse direction,
and it is harder to find a tool for repairing GPT partition Tables,
and you have almost 400 MB consumed in GPT tables.

I am very willing to learn how GPT fault tolerance can deal with an accident to GPT Tables.

4. No clashes in disk type IDs
5. Clashes are definitively solved with GUIDs

That was NOT my experience under 64 bit Windows 7 Ultimate + SP1,
when normal start-up from cold allocated
Disk 0 W.D.C. 600 GB style GPT
Disk 1 Samsung 930 GB style MBR
Disk 2 OCZ 56 GB style MBR (primary system drive)

After countless visits from power-on into BIOS and then to Windows and a successful launch,
my next attempt resulted in the WDC having its complicated GUID replaced by the same 8 hexdigit ID as the Samsung,
After THAT stupidity Windows compounded its errors by WRONGLY choosing to put the Samsung OFFLINE.
Consequently :-
The WDC drive was useless and out of action because Windows saw an MBR style Disk ID that had no valid partitions,
The Samsung drive was useless because Windows had put it OFFLINE,
and as a result Windows lost access to the partitions that held "Pagefile.sys" and %TEMP%

I have not yet found any tool that will change an MBR ID into a GUID without requiring me to initialize the WDC and losing all the data.

and the fact that some of these tools disable GPT capability on freeware versions is a PRICELESS benefit of MBR.

Which? Not any more.

Both Minitool and Macrium had freeware versions restricted to MBR Disks a couple of years ago.
I agree those two products have recently removed those restrictions,
but I thought it probable that their competitors might either be in the process of catching up and not yet there,
or their competitors may have "accelerated" development and left the Beta testing for unsuspecting users.
Sorry, but thirty years of working in software and systems development has made me a little cynical.

Regards
Alan

 

[milindsmart]

Under XP I discovered that when CMD.EXE invoked REG.EXE to READ (not modify) a key
and piped both STDOUT and STDERR to the same log file,
for keys that had sub-keys that had no access there would be simultaneous answers on both output streams for the same interrogation,
and MOSTLY both streams were logged, but sometimes neither were logged, and at other times only one was logged but not the other.
XP was successful better than 99% of the time when my batch script and the output log file were in partition C:\
When I moved only the log file destination to a partition at the other end of the HDD,
the success rate fell below 90%.
The problem was perfectly cured by disabling the output cache
(I think it may have been called "Enable Safely Remove")
Perfect cure - but Windows started up 3 times slower.
You are right - XP sucks.

Holy crap... I did not know MS were capable of making such mundane stupidities in their software design.

An MBR has only two tables, I think they are called Primary and Extended
Break the primary and you lose everything.
break the extended and you lose all the Logicals.

You're right about the MBR... But the extended partition contains logical partitions that are linked in a chain, not kept together in the extended boot record as any sane person would have done... Of course, if the complete disk information was stored in each place, then it would have been very smart I suppose, but this is pretty stupid, as you have said below :

BUT you have only two such tables to be broken, and as of 9 months ago the Professional licensed version of Minitool Partition Wizard was NOT capable of REPAIRING a broken GPT Partition Table,
even though it could shrink and extend GPT partitions.

I understand that every GPT partition is preceded by its own 3 MB GPT Table and that one GPT table links to the next GPT Table,
which MIGHT be the complication that Minitool Repair could not cope with.

If you have 128 partitions :-
Using MBR you have only 2 tables that may be broken,
(and 4 or 5 years ago freeware Mintool version 4.? fixed my Laptop which had lost ALL 7 partitions.)

Using GPT you have 128 tables that may be broken, and that loses all the subsequent partitions,
unless of course there are "redundant" backward pointing links to traverse the chain in the reverse direction,
and it is harder to find a tool for repairing GPT partition Tables,
and you have almost 400 MB consumed in GPT tables.

Except that this is NOT how GPT works. At ALL. Wiki. IBM Developer works. This is more like MBR works.

There are ONLY 2 partition tables, one at the beginning, one at the end.

I am very willing to learn how GPT fault tolerance can deal with an accident to GPT Tables.
That was NOT my experience under 64 bit Windows 7 Ultimate + SP1,
when normal start-up from cold allocated
Disk 0 W.D.C. 600 GB style GPT
Disk 1 Samsung 930 GB style MBR
Disk 2 OCZ 56 GB style MBR (primary system drive)

After countless visits from power-on into BIOS and then to Windows and a successful launch,
my next attempt resulted in the WDC having its complicated GUID replaced by the same 8 hexdigit ID as the Samsung,
After THAT stupidity Windows compounded its errors by WRONGLY choosing to put the Samsung OFFLINE.
Consequently :-
The WDC drive was useless and out of action because Windows saw an MBR style Disk ID that had no valid partitions,
The Samsung drive was useless because Windows had put it OFFLINE,
and as a result Windows lost access to the partitions that held "Pagefile.sys" and %TEMP%

That's extremely stupid behavior by windows... Now you're giving me doubts :shock: nevertheless, it can be fixed.

I have not yet found any tool that will change an MBR ID into a GUID without requiring me to initialize the WDC and losing all the data.

Have you tried using the advanced functionality present in gdisk ? I know some little about all this, and I could help you in some of this, but of course, better experts should be around here..

Both Minitool and Macrium had freeware versions restricted to MBR Disks a couple of years ago.
I agree those two products have recently removed those restrictions,
but I thought it probable that their competitors might either be in the process of catching up and not yet there,
or their competitors may have "accelerated" development and left the Beta testing for unsuspecting users.
Sorry, but thirty years of working in software and systems development has made me a little cynical.

Yeah well that is possible. And 30 years in software and systems... well, I'm not sure I can say anything at all after reading this, because I have zero... Perhaps my optimism is unwarranted. However, the spec doesn't seem to have any obvious errors. Other than that, I can just say I also try to gather my info from sources that list detailed information, and verifiable testable results, but can't be completely sure... Like for example, never heard of these cases you just said.... what were MS thinking? :-/

 

[mjf]

@Alan10
I think your original question is a good one that I raised in the VIP area. I didn't get any feedback but I personally tend to agree with the views of TVeblen. I run a UEFI BIOS and recognize its significant benefits over the older BIOS. However, I choose to retain an MBR boot drive at the moment. Why - I boot to an SSD 128GB (well under 2.2TB!). My internal data HDDs are 1TB and I only require 2-3 partitions. I am uncertain about the compatibility of some of my software with GPT formatted drives.

 

[milindsmart]

What kind of software is this that worries about your partition type? Partitioning software? If you aren't using paid software then refresh your software collection.

Also, "UEFI BIOS" is better expressed as "UEFI firmware".

 

[mjf]

What kind of software is this that worries about your partition type? Partitioning software? If you aren't using paid software then refresh your software collection.

Also, "UEFI BIOS" is better expressed as "UEFI firmware".

Not so long ago free Macrium Reflect would not work on a GPT partitioned drive - that's one example.
MS state:
" Legacy software that does not know about GPT interprets only the Protected MBR when it accesses a GPT disk. These tools will view a GPT disk as having a single encompassing (possibly unrecognized) partition by interpreting the Protected MBR, rather than mistaking the disk for one that is unpartitioned." Source.
I guess this is not much of an issue for many people and you could argue stop using legacy software that wants to access an MBR partition table.
The reference describes the benefits of GPT but they don't have an impact on me at the moment so I decided to stay with MBR for the time being. If I wanted a > 2.2TB internal data drive then it would be GPT but I can never see myself wanting to boot to one.

On my ASUS Motherboard box it has blazon in bold UEFI BIOS as opposed to Legacy BIOS. Of course the BIOS is firmware so by all means refer to it as UEFI firmware if you like

 

[milindsmart]

It's not an economically justifiable shift, from MBR to GPT, right now, but you'll want to consider just how many different implementation quirks have been shoved into a "specification" for MBR to work. It's stable and working to the extent it is because of a large number of workarounds implemented in every little low level program that accesses disks. Not robust by design.

Why would you want to use MBR for the sole purpose of using an old tool? What if I told you 1 bit unexpected corruption could potentially cause your entire partition to become useless? Does MBR still seem like a great default system?

It's the equivalent of your surgeon being a decrepit old man/woman with shaking hands and stinking white(?) coat. Sure xe may be more familiar. But do you think xe's reliable? No problems till you need a surgery, but what might happen during one?

 

[Computer0304]

Thanks.

My MoBo can only boot MBR but can access both programs and data partitions on my GPT HDD.
My largest existing HDD is 1 GB.
Is it probable that my existing 4 year old MoBo and BIOS would fail to deal with a 3 GB partition or even a 3 GB HDD,
regardless of whether it was MBR or GPT style ?

Supplementary Question on (Extended) Partition Tables :-
Are all such tables read from all HDDs upon start-up and the Partition Letters and MFT/Directory etc. locations copied to RAM,
or does the Operating System trawl through these tables on each HDD each time I try to access an item in partition F:\ ?

Regards
Alan

You meant 1tb right?

 

[mjf]

What if I told you 1 bit unexpected corruption could potentially cause your entire partition to become useless?

You wouldn't be telling me anything I didn't know. It applies to lots of things.
I'm quite familiar with the MBR and the partition table structure. Besides I back up the MBR, mainly because of the partition table, and can restore it whenever I like. I also keep numerous system images.
When you can say a GPT is going to make my system work better then I may bother to convert. This would involve a clean install and I have a lot of paid software that would need reactivating. Reactivating the software may involve headaches.

 

[milindsmart]

Like I said, it's not something you can justify for an audit. It just vastly decreases the probability of ambiguity when modifying partition tables.

 

[alan10]

In my opinion it can be dangerous for a Partition Manager to modify the partitions of the drive that holds the live Windows C:\ partition which it is running under.
Safer to use a Boot CD.

Question :-
Is it safer to use a Boot CD from a third party that has :-
a newly developed (i.e. Beta release) GPT capability on a drive with an allegedly "less ambiguous" Fake MBR plus GPT partitions;
or well established and finely honed capability on a drive with MBR partitions ?

There are tools that can make and restore image backups of "Track Zero",
which I believe holds the MBR and the primary and extended partition tables,
so hopefully it is possible to promptly correct any sort of disaster when altering MBR partitions.

Is it possible to promptly correct any sort of disaster when altering GPT partitions ?

Regards
Alan

 

[milindsmart]

It's not "beta" at all. It's been around for about ten years now and defined before 2000s.

What do you have against the "fake" MBR? It's a partition covering the entire interior of the disk, and ANY TOOL that does not understand GPT will refuse to work with it. 0xEE is unknown for all pure-MBR tools. It's not fake except in a VERY narrow sense.

Besides, I have had serious problems on computers of 2009-10 with MBR partitions with tools like diskpart AND gparted. Five partitions had been created somehow, and I have no idea which other partition got sacrificed. No more "finely-tuned" than GPT tools. Other than that, witness the steady stream of problems people have even today with partitioning : a direct consequence of how badly MBR was designed and how precariously it has been made to work today.

Again, Track 0 DOES NOT hold EBR and logical partition table. They start from the beginning of the Extended Partition and CHAIN each logical partition to the next in LINKED-LIST style. So no, Track 0 backup CANNOT restore any of your logical partitions.

Yes you CAN take Backups of GPT. It's just as simple as that of MBR, take 1st 34 sectors backup. One such backup is stored at the end of the disk, COMPULSORILY. Only thing to be careful about is to not use sector by sector copies of GPT disks, the partition GUIDs must be set to zero.

 

[alan10]

"Beta Release" refers to the first generation/issue/release of a Partition MANAGER.
I was not applying the term "Beta Release" to the definition of GPT Partitions.


I have nothing against the "Fake MBR" other than it is an extra detail to understand.
Please note that I was not being derogatory by calling it a "Fake MBR",
I was just using the term which I once saw in a technical article.


You might have experienced a loss of an MBR partition when using Diskpart and Gparted.

I have lost ALL GPT PARTITIONS on a complete W.D.C. drive due to Windows 7 incompatibility with GPT drives connected via SATA.

Microsoft have published a "USER BEWARE AND DEAL WITH IT" type a KB????,
warning that they are NOT going to deal with their own stupid mistakes.
When physical drives are connected via SATA,
Windows Disk Management allocates drive numbers from 0 to (number of physical drives-1)
in "order of enumeration", and NOT in a sensible fixed basis of physical port numbers etc.

The consequence is that my physical drives are numbered after a cold start-up as
W.D.C. GPT HDD = 0
Samsung MBR HDD = 1
OCZ MBR SSD = 2
That numbering is changed by a RESTART when Windows starts up and finds the HDD's are running at speed.

Windows is full of race hazards.
I guess that when I did a cold start-up and BRIEFLY examined a BIOS setting before allowing the boot-up to continue into Windows,
the drives were at partial speed and as a result of some race hazard,
Windows REPLACED the W.D.C. GUID with an exact duplicate of the Samsung 8_Hex_Digit Disk Identity,
and then complained about having two disks with the same Identity,
and then put the Samsung OFFLINE.

The W.D.C. GPT Disk was seen as MBR style with a couple of RAW partitions and everything else was alleged to be "Unalllocated Space".

I was able to put the W.D.C. OFFLINE and then make the Samsung ONLINE and the Samsung has not lost any data.
There seems to be no way that I can replace the incorrect 8_Hex_Digit Disk Identity with an appropriate GUID,
and no partition manager is able to deal with this horrendous situation because Windows insists it is an MBR drive.

Freeware LAZESOFT has rescued all the files that were ever on the W.D.C. when it had been seen as a GPT style.
Unfortunately this also includes files which I had recently deleted but which had NOT been over-written with more recent files.
Lazesoft Recovery Suite Home Free Download

LAZESOFT has also shown there seem to be 3 MByte partitions between the partitions that I knew were on the drive before the catastrophe.
I had assumed that each of these 3 MB partitions held another link in the link-list of GPT partitions.
Whatever they are - they are another mystery.

Summary of my experience :-
MBR partitions all immune to catastrophic disaster,
GPT partitions very susceptible and NOT easily repairable.

My decision :-
Never again have a mixture of GPT and MBR drives connected via SATA.

Regards
Alan