Hello All; There are many ways to accomplish this, but I want to thank Milindsmart for suggesting the concept and idea. I am now dual booting Win 7 and Win 8.1 Pro on a GPT 3.4 TB array on an old P55 BIOS MBR system. Basically, as outlined, you need 2 separate disks- either 2 hard drives or USB stick and HDD as suggested. I tried it with the USB stick first - worked great. Just copy a Windows 7/8 hidden BOOTmgr file into the root of the stick. Then copy the hidden Boot folder into same. Now you need to point (edit) your BCD file (in the USB stick's Boot folder) to your Windows installation(s) on the GPT disk. The old MBR BIOS reads the boot info from the USB stick, presents a boot menu (in dual boot configurations) and you select which OS on the GPT disk to boot to and all is good. To edit the BCD (boot configuration data), I used EasyBCD 2.2 It's a graphical (and much easier) BCD editor. To get my system to dual boot both Win 7 and Win 8.1, I had to copy over the updated Windows 8 BOOTmgr file into the root of the USB stick, otherwise, 8 won't load. The 7 BOOTmgr doesn't correctly recognize the newer Win 8 OS while the 8 BOOTmgr will recognize and boot older windows OS. Now to transfer all of this over to a second hard drive in the system so you don't have to keep using the USB stick to boot to your large GPT disk, although truthfully there is no noticeable performance hit booting from the USB stick). After the OS loads, you can actually eject the USB stick and use it normally to carry with you and transfer data between computers. Problem is - don't loose it! or have another computer load a virus onto it which will corrupt your boot process. A cleaner, more stable (and safer) solution is with a second HDD that is permanently installed in the computer.
1st: The second drive has to be formatted in the old MBR partitioning scheme (which it most likely already is if in an older computer). It can be any old drive of any size. If you have an old working drive laying around, use it. If you already have a second data HDD, use it.
2nd: If there are already partitions and data on the drive, you will have to do some resizing and moving of partitions. Use any of the excellent free Partitioning software out there. I use EaseUS PM. You can also use the built in Windows or
command line DiskPart. Basically, you want to free up about 100mgs of free space (unallocated) at the very beginning of the drive. This will be where the MBR boot files will eventually reside. Now create partition in and format that 100 mgs free space you created at the beginning of the HDD. You can name it System Reserved and set the 0x7 flag (DiskPart) so the BIOS will know to boot from it or you can use EasyBCD to deploy (copy) MBR to the new partition. Once the new partition is bootable, you will need to edit the BCD to point it to the Windows installation on your GPT disk. EasyBCD works great for that as well. Don't forget that for a partition to be bootable, it has to be set as the ACTIVE partition. If you use EasyBCD to Install an MBR in a partition, it will also mark it as ACTIVE. If not, you can use Windows Disk Management, DiskPart, or other partitioning software to set the partition as ACTIVE.
Let's take a hypothetical, but I think relatively common situation. You have one older BIOS MBR based computer that boots Win 7 (64 bit). You have just bought a new 3 TB HDD. Maybe your older HDD with Win 7 on it is getting slow or is old and might be failing. You want migrate your OS over to the new 3 TB HDD you just bought, but also want to use all of the space and not be limited to 2.2 TBs by the MBR partition scheme. This means formatting the disk as GPT partition scheme. Problem is, older MBR BIOS based hardware won't boot to a GPT disk. Create the GPT disk, then migrate your current OS over to it, using several possible programs (EaseUS PM has a neat 'migrate OS to HDD/SSD' feature). You can also make an image of your OS(s) and restore it to the new GPT disk. Macrium Reflect will allow an MBR image to be restored to a new GPT disk.
Before migrating/imaging your OS, I suggest a thorough clean-up of it using Disk Cleanup or other. I also suggest installing all the latest Windows/virus updates. Uninstall any programs you don't use. If on a HDD, defrag it. Not necessary if on a SSD.
The point is you need to get/copy your Windows onto the GPT disk in a DATA partition. I imaged my dual boot (7 and 8.1 Pro) using Reflect, then restored the images to my new 3.4 TB GPT array. I bought a fast 250 Gig SSD and migrated/imaged the OSes over to it. This is now my main boot drive. It is, obviously, MBR. I've retired the 3 year old original OS drive. It is labeled and safely stored as my fail safe start up OS drive. The 3.4 TB GPT array is my 'working' OS back-up. The two OSes only take up <230 Gigs, so the rest of the 3.2 TBs is data/videos. etc. Like I said the SSD is my fast boot up drive. I simply added the two back-up OSes on the GPT disk to the boot menu on the MBR SSD. When booting, I'm presented with 4 choices. Win7 SSD, Win 8.1 Pro SSD (default) and Win7 GPT array and Win 8.1 Pro GPT array. If the OSes on the SSD fail or become inoperable/corrupted (unlikely, but possible), I can boot to one of the back-up OSes on the GPT array. If the SSD fails completely (unlikely, but possible), I can use the USB stick to boot the OSes on the GPT array. What is the point of all of this you say? Despite its age, this is my working computer that I make my living on. I need it to be available, working and reliable 24/7 and it stores a ton of video and data (thus the 3.4 TB array). With this configuration, I will always be able to boot into a working reliable OS. Sometimes life is good.