Windows 7 Forums
Welcome to Windows 7 Forums. Our forum is dedicated to helping you find support and solutions for any problems regarding your Windows 7 PC be it Dell, HP, Acer, Asus or a custom build. We also provide an extensive Windows 7 tutorial section that covers a wide range of tips and tricks.


Windows 7: what exactly happens when you boot with two bootable drives?

3 Weeks Ago   #1
johnhoh

Win7 pro x64
 
 
what exactly happens when you boot with two bootable drives?

Here is what has been my experience.

If you clone a disk then immediately restart with both disks still connected, whichever disk is NOT booted from usually loses its ability to boot from then on, presumably because the booting disk messes with the other disk's boot sector (?).

However, if after cloning you immediately shut down and disconnect the original drive and then reboot from only the new drive, then shut down and disconnect it and reconnect the original boot drive and boot from only it, and THEN connect both drives, both drives will remain bootable and you have a poor-man's multiboot system, i.e. one where there is no windows multi-boot menu however you can simply select which drive to boot from by using the bios boot menu.

This post is a question as to exactly what is happening in the above scenarios. Feel free to correct anything wrong in the above as well.


My System SpecsSystem Spec
.
3 Weeks Ago   #2
SIW2

Microsoft Community Contributor Award Recipient

Vista x64 / 7 X64
 
 

Quote:
if after cloning you immediately shut down and disconnect the original drive and then reboot from only the new drive, then shut down and disconnect it and reconnect the original boot drive and boot from only it, and THEN connect both drives, both drives will remain bootable and you have a poor-man's multiboot system
Have you done that often?
My System SpecsSystem Spec
3 Weeks Ago   #3
dg1261

Windows 7/8.1/10 multiboot
 
 

Results depend to an extent on how complete the "clone" is. Most utilities don't actually make *exact* clones because of the below issues, so they make close enough copies with a few tweaks based on guesses as to what your ultimate goal is.

Windows differentiates logical volumes by assigning a unique "signature" to each partition, derived from a disk identifier and a partition identifier.* Those signatures are what you'll see recorded in the [HKLM\System\MountedDevices] registry key, and which has been a focus of discussion in another thread.

Partitions can be easily differentiated simply by their location on disk, so the starting byte location is used as the partition identifier.

Disks are differentiated by writing a 4-byte "DiskID" in the first sector of the hard drive.** It's important to note this is not part of the firmware and is not put there by the disk's manufacturer. It's just a pseudo-random string of bytes written by the OS in order to differentiate otherwise similar hard disks. This means a system should not have two disks with the same DiskID. If that condition is encountered, Windows will rewrite a new DiskID on the second disk. (It's a random string, so if you were to do it manually, you're free to use any byte values you want.)

I've covered further details on my webpage, which may help you resolve the circumstances under which you'd encounter the symptoms you describe. My page was written during the Win2000-XP era, but parts of it are still relevant because the same methodology is still used in Windows 10.

"... whichever disk is NOT booted from usually loses its ability to boot from then on ..."

When you encounter that situation, you should be able to repair it by removing the [MountedDevices] entries, removing the second disk, and booting it as a single-disk system. Windows should regenerate the partition sigs and commit them to its registry, after which the second disk can be reintroduced into the system.

In a nutshell, the key is whether the OS being booted has had a chance to establish valid registry entries for its own System and Boot partitions.



* This part-sig method predates the rise in popularity of the GUID system, but I have on occasion noticed a few recent systems that use GUIDs in the registry's DosDevices entries. I haven't investigated when or why that occurs, but it's worth noting that the part-sig identifiers may no longer be the only game in town.

** The DiskID dates back to Windows NT and was originally called the "NT Serial Number". There are actually 6 bytes reserved for its use, but in all cases I've seen, only 4 bytes are used. These 6 bytes are at location 0x1B80 in the disk's first sector -- aka, the MBR boot sector.
My System SpecsSystem Spec
.

3 Weeks Ago   #4
SIW2

Microsoft Community Contributor Award Recipient

Vista x64 / 7 X64
 
 

I have a module on the little thing I made. It was intended for PE , however, most modules work fine in windows too.

You can fix the problem by pointing this at the non booting windows partition

NT6REPAIRx86.zip

what exactly happens when you boot with two bootable drives?-nt6rep-fix-driveletter.jpg


My System SpecsSystem Spec
3 Weeks Ago   #5
johnhoh

Win7 pro x64
 
 

Thanks for that thorough explanation. Let me know if I am describing the issue correctly, in the situation of the unbootable clone.

You have two disks, A and B. Each are one partition. Disk B is blank. You clone A onto B. If you then remove A and boot from B, it boots no problem. But if you do not remove A, and instead reboot from A (with B still connected), B may become unbootable. Is this because upon that post-clone reboot, windows overwrites B's Disk ID with a new one? Because windows must be writing something onto B that makes it unbootable
My System SpecsSystem Spec
3 Weeks Ago   #6
SIW2

Microsoft Community Contributor Award Recipient

Vista x64 / 7 X64
 
 

It will if there is clash of disk signatures.
My System SpecsSystem Spec
3 Weeks Ago   #7
johnhoh

Win7 pro x64
 
 

Quote   Quote: Originally Posted by SIW2 View Post
It will if there is clash of disk signatures.
Does NT6REPAIRx86.zip change the registry or does it change the disk ID?
My System SpecsSystem Spec
3 Weeks Ago   #8
SIW2

Microsoft Community Contributor Award Recipient

Vista x64 / 7 X64
 
 

It changes the registry.

what exactly happens when you boot with two bootable drives?-dosdevs.jpg


My System SpecsSystem Spec
3 Weeks Ago   #9
SIW2

Microsoft Community Contributor Award Recipient

Vista x64 / 7 X64
 
 

Just booted into the target OS and the entries are there:

what exactly happens when you boot with two bootable drives?-capture-dosdevs-repopulated.jpg


My System SpecsSystem Spec
3 Weeks Ago   #10
dg1261

Windows 7/8.1/10 multiboot
 
 

Quote   Quote: Originally Posted by johnhoh View Post
You have two disks, A and B. Each are one partition. Disk B is blank. You clone A onto B. If you then remove A and boot from B, it boots no problem. But if you do not remove A, and instead reboot from A (with B still connected), B may become unbootable. Is this because upon that post-clone reboot, windows overwrites B's Disk ID with a new one? Because windows must be writing something onto B that makes it unbootable
It's hard to say precisely because different tools tweak things differently, so it's hard to know how your tool is doing it.

For instance, some tools will duplicate the DiskID on the assumption you're going to remove the source disk and immediately replace it with the original and thus there won't be a DiskID conflict. In such an instance, yes, rebooting from the source disk will force a DiskID change, and in effect subvert the tool's carefully laid plans.

Other tools will perform the same task by using a new DiskID for the target disk, and make up for that difference by tweaking the registry entry so it uses the new DiskID in place of the old -- in which case, note the registry entries won't be an exact clone of the source.

If you're curious exactly how your tool does it, you might try comparing the DiskIDs and registry entries on disk B in the three scenarios: right after the clone is made but no reboots yet; after cloning and rebooting from A; and after cloning/removing A/booting B.
My System SpecsSystem Spec
Reply

 what exactly happens when you boot with two bootable drives?




Thread Tools




Similar help and support threads
Thread Forum
HP Pav - HDD trouble. BIOHD-3 error. No bootable drives detected.
PC started freezing up. HP Pav with Win7 Home Prem (w/o SerPk 1).Performed diagnostic from boot screen by pressing F9 key. CPU, Mem, Hdd all passed except got "BIOHD-3 error. No bootable drives detected", yet it was still booting up.:confused: Within minutes though pc would get stuck/freeze with...
Installation & Setup
Problem converting two bootable drives to one
A few months ago I swapped out one of my drives for a new SSD and installed Windows 7 on it, installed all programs, configured everything, and gradually moved data over while still using the old one to get work done during the transition. I'm at the point I'd like to wipe out the old drive and...
Installation & Setup
Cannot boot from a bootable CD through USB
I can't boot from a separate CD that is connected with USB to too laptops: DELL running WIN 8.1 and a Lenovo S400 running WIN 7. i tested the CD in an old XP machine with an internal CD and it worked. I think once my DELL booted from an other bootable CD using USB but i am not 100% sure. I set...
Backup and Restore
Running two bootable drives makes one permanently unbootable?
I am just finishing a clean reinstall of EVERYTHING onto a new drive. I had hoped, once I'd installed Win 7 Pro x64 in the new drive, I'd be able to put the other drive back in the system to transfer certain files. But every time I did, it rendered the new drive unbootable, even after I'd taken...
General Discussion
bootable dvd cant boot even though bios configured
This is strange to me. I have 3 laptops, was trying to boot from linux distro 32 bit on all of them. The oldest toshiba laptop is a 32 bit and the bootable dvd works ! My 2 newer Asus laptops 64bit cant boot the dvd. What gives? All of their boot orders have also been configured. Asus even...
Hardware & Devices


Our Sites

Site Links

About Us

Find Us

Windows 7 Forums is an independent web site and has not been authorized, sponsored, or otherwise approved by Microsoft Corporation. "Windows 7" and related materials are trademarks of Microsoft Corp.

Designer Media Ltd

All times are GMT -5. The time now is 04:23.
Twitter Facebook