Super User Asked by Rule184 on December 3, 2021
I found out that an old desktop I had is still working. Last time it was used it had Windows 10 installed, although the hardware is almost 10 years old. I decide I want to have a dual boot with Windows 10 and Debian 10. I have a bootable pen drive with Debian 10 I used last month on my new laptop, so I stick it in. I can’t boot from the USB. I read somewhere I should use switch from BIOS to UEFI to boot from USB and that I should use the tool mbr2gpt for that, since there’s no UEFI option in the BIOS settings. I open a command line in Windows and run:
mbr2gpt /validate
mbr2gpt /convert
When I restart the computer, nothing boots anymore, it just goes to a black screen.
edit: It seems I should have checked if the system supports UEFI before using the mbr2gpt tool, but now it’s done.
There is no really supported way of going back from UEFI to MBR. You can probably recreate the MBR partition table and repurpose the partitions as normal MBR partitions, but the BIOS boot code needed in sector 0 and in the boot partition (which will have been rewritten into a EFI System Partition) is not there anymore. Apart from reinstalling, I wouldn’t know where to begin.
About the USB key not being able to boot on that computer: does the key not appear in the boot selection at all, or does it appear, but when booting from it, you get a "No bootable device" or "Operating System not found" (or similar) message? How did you create that bootable Debian 10 pen drive? By dumping a Hybrid ISO/FAT image on it, or using some other tool? With that info (or a link to the ISO/FAT image in question), one could determine if the USB key should be able to boot regular BIOS or if it is UEFI only.
Answered by Ro-ee on December 3, 2021
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