Convert GPT Disk to MBR Disk in Windows 10  

urbanman2004 said:

I then convert the newly duplicated disk to MBR. After doing so I would connect the SSD to another system (BIOS, not UEFI), but that system would give me a booting error.

You need to do more than convert the GPT drive to MBR to get it to boot under legacy BIOS. You need to have a system partition that is marked as ACTIVE on the drive that contains the boot files specific to legacy BIOS booting.

NavyLCDR said:

You need to do more than convert the GPT drive to MBR to get it to boot under legacy BIOS. You need to have a system partition that is marked as ACTIVE on the drive that contains the boot files specific to legacy BIOS booting.

True, which I forgot to mention I had done that step within the app during my first attempt which is what I planned to do on my second attempt as well.

SO I was successfully able to convert my Windows 10 OS from GPT to MBR without data loss... I'm already very familiar w/ how to do it the other way around, done it multiple times (which I consider more difficult).
Here's what I did:
01. Using "AOMEI Partition Assistant Pro Edition" on a GPT/UEFI PC -> Migrate to OS -> select prepared "clean" spare disk drive (in USB enclosure), untick 'Make boot structure same as original drive [producing an MBR instead of GPT] (can't remember exactly what it said verbatim)', and apply changes
02. After following the on-screen prompts, it will automatically restart PC in a Windows Preinstallation Environment (or PreOS Mode) to complete the task.
03. Manually restart from original disk drive, run "AOMEI Partition Assistant Pro Edition", select duplicated disk drive -> Wipe 'EFI' partition -> Merge [blank] partition (with OS partition), and apply changes
04. Extract duplicate disk drive from USB enclosure then move drive and connect it to assigned PC (MBR/BIOS)
05. Start MBR/BIOS PC by using Windows 10 boot disk, select next "Next" -> Repair your computer -> Troubleshoot -> Advanced options -> Command Prompt, and use the following commands
* bootrec /fixmbr
* bootrec /fixboot
* bootrec /scanos (It should show "C:/ Windows [1]")
* bootrec /rebuildbcd
06. Using "bootrec /scanos" should show "Total identified Windows installations: 0" confirming it worked.
07. Restart the PC, and you should be greeted with a Windows logo screen displaying "Getting devices ready" below it indicating you are now able to successfully boot into the duplicate drive.

Before:

After:


Sources:
Migrate Windows OS from GPT Disk to MBR SSD without Reinstalling
How to repair Master Boot Record (MBR) on Windows 10 Pureinfotech

I used EaseUS Partition Master Technician Edition to convert a Win 10 OS HDD and it said compleated sucessfully however on boot i got..

BOOTMGR is compressed
Press Ctrl+Alt+Del to restart

And that was all... how would i safely go about decompressing or repairing the MBR?

DaKaiHigh said:

I used EaseUS Partition Master Technician Edition to convert a Win 10 OS HDD and it said compleated sucessfully however on boot i got..

BOOTMGR is compressed
Press Ctrl+Alt+Del to restart

And that was all... how would i safely go about decompressing or repairing the MBR?

You'll need a Windows 10 installation USB flash drive to boot from. If the drive has a separate system partition it is supposed to be booting from, I would recommend deleting that partition. Recreate a primary partition, format it as fat32, mark it as active, assign it a drive letter. Then from the \boot folder on the Windows 10 installation USB flash drive, run bootsect. For example: bootsect /nt60 all /force /mbr. Then you will want to run the bcdboot command with the proper options to write the BCD to the new system reserved partition.

If the drive does not have a separate system partition, then I recommend shrinking the OS partition to make room for one and creating one. You can perform the above commands on the OS partition, after you mark it as active, and boot from the OS partition itself, but I don't recommend it.

The drive I am dealing with is listed here as Disk 2(MBR) with 2 primary partitions and an unallocated logical!

DaKaiHigh said:

The drive I am dealing with is listed here as Disk 2(MBR) with 2 primary partitions and an unallocated logical!

It depends on what the 2 primary partitions contain.

- - - Updated - - -

C: is the OS
F: Recovery
System Volume Information

On disk 2, I would create a primary partition in the 100 MB unallocated space, format as FAT32, mark it as active, assign it a drive letter, then run the commands from a Windows 10 USB flash drive to write the boot loader and BCD files to it.

I'm signing off for the day....have to go to work. Darn jobs.