I have the following situation:
I have a "backup" of a failed/non more existend disk which has an exact copy of all its partitions but the difference that it has no bootloader or what so ever and it uses GPT while the original disk used MBR. The partitions (info from GParted and diskpart) are
1: NTFS | System-reserved | 350MB | boot, esp | no drive letter assigned
2: NTFS | | 232GB | msftdata | C:
3: NTFS | | 450MB | diag | no drive letter assigned
Now booting from this disk won't work. That was to expect as the bootloader was on the original disk. But I'm having a really hard time to recreate it and get the disk to boot. I tried a lot with diskpart and bootrec but it seems that the disk would need to use MBR as well for bootrec to work and I haven't found a way to convert it to MBR without loosing the data.
So the question is, how can I boot from this disk into my Windows 10 installation on C: again?
PS: I'm pretty sure this was answered in a similar way before as it seems quite basic so a link to a solution would also be great as I am not able to find one…
Best Answer
Windows ties its boot mode to its partition table type: Windows boots from MBR disks only in BIOS mode and from GPT disks only in EFI mode. Thus, to boot from your disk as it is now, you must install an EFI-mode boot loader. This also means that your computer must have EFI firmware (as do most machines introduced since mid-2011). Here's a blog post that describes how to do the conversion:
http://sdnalloh.com/converting-win7-from-mbr-to-gpt/
Most of that is already done (or moot) in your case; you really need to do only the stuff on the second half or so of the second page.
Alternatively, you could do an in-place GPT-to-MBR conversion and recover in a traditional BIOS/MBR way. This might be the better way to go if you've got an older BIOS-based computer; but if you need GPT because the disk is bigger than 2TiB, this would be a poor option.