Dual Boot 7&10 on BIOS with GPT?

I'd posted this on another forum, but didn't receive much attention, so I figured this forum was a better choice. I'm sorry if this is an out of bounds cross-post!

I do see that there is a couple of similar questions here, but not this one specifically.

I learned about trying to boot GPT from BIOS here:

Booting Windows 7 from a GPT Disk Using BIOS (non-UEFI) | Windows OS Hub

And came up with this idea:

(begin cross post...)

Hey all!

I have a dual boot system that I just had to restore a backup of the Windows 10 install and I put it on a 4tb disk. Of course, it turns out that now I can't expand the drive to the full 4tb size because it's a BIOS system.

Now , I (believe) that I've learned that if I first boot to a MBR disk, that I can then boot to a GPT disk?

The Windows 10 is actually an upgrade of the Windows 7 install (via Windows 8) that I'd cloned before upgrading. They're on separate Disks and both have BCD Stores on them with Boot Menu's for both installs.

If I boot to the Windows 7 Disk and select Windows 7, it boots fine. If I select the Windows 10 install, I get the Can't Verify Signature error (0xc0000428?) ( I haven't yet converted the Windows 10 Disk to GPT).

If I boot to the Windows 10 Disk, I can load both the 7 and 10 installs.

So, my 2 questions are: Will my plan to convert the 10 Disk to GPT work (by booting to the MBR Disk first) and how do I fix the dual-booting from the 7 disk?



I've additionally (maybe) learned that I can fix the 0xc0000428 issue by copying the Windows 10 disk BOOTMGR to the Windows 7 disk? I'm about to give it a try, but I'm getting bored with copying the 300gb backup back to this 4tb disk and would like to avoid it again.

What will that do on boot? What's the best way to attempt to get it to boot 1st try?

Also, if I convert the (current) 2tb MBR partition to GPT, is converting it back to MBR just a matter of the CONVERT command again? Assuming I don't expand it over the 2.2tb limit? I have located a utility that is supposed to convert to GPT without data loss (Aoemei). I'm unsure if the same utility will convert back.




Knows where his towel is.
Staff member
You can only boot W7 from a GPT disk under UEFI
Windows and GPT FAQ - Windows 10 hardware dev
You can't boot W8/8.1/10 using the W7 bootmgr (newer winload.exe digital signatures aren't seen as valid because older OSs aren't forward compatible)
You must always boot using the latest version of bootmgr you own (i.e. W10's in your case) because it is backward compatible with all older versions of winload.exe.
You can workaround the restriction if you have a pressing need to boot from the W7 drive by replacing bootmgr on W7's root with the (larger) version from W10.
Thanks for the reply.

I did successfully accomplish the task.

I can now dual boot to Windows 7 on the 1tb disk and Windows 10 on the 4tb disk fully extended.

I think after fussing with EasyBCD in Windows 7 and then booting to WinRE on the Windows 10 (64 bit) USB drive I was able to choose 'Startup Repair' which did replace the BootManager. I was prompted to choose which OS I wanted to perform Startup Repair on. Initially, I chose Windows 7 and on the 2nd try I chose Windows 10.

After converting the 4tb disk to GPT in Windows 7 using Aoemei I had to boot back into 7 as I received another Winload error (a different one) and use EasyBCD to create another Boot Menu entry, which worked!

So, the answer is 'yes' you can do it that way!

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