I have just installed Debian 10 (Buster). It worked fine, until I did a reboot. Now it will not boot.

I re-loading installer. When I try to re-install grub, it wants me to re-partition the disk (even though this is fine). So I go into manual partitioning, and set thinks up as before (and same as Debian 8). It says I have 1MB free before 1st partition, it won't let me set the boot flag on any of my partitions.

Then I am told that the target file-system contains files that may be a problem (yes I know, I just want to get to the grub-installer). debootstrap fails (good). Select Grub-install again, am told that it depends on earlier steps, I select back. I am now in grub installer.

I given option: Enter device manually, I select it.
I accept /dev/sda

I get an error: `Executing 'grub-install /dev/sda' failed. This is a fatal error.

I select continue and re-try with /dev/sda1

I now get an error it seems that this computer is configured to boot via EFI, but …

EFI is disabled (well as close to disabled as is possible: “legacy” enables, and “legacy first” selected in boot options) in bios setup. EFI dose not work: It would not boot the install media. It may have been enabled during install, but with bios-boot selected from boot menu. This system previously had Debian Jessie installed. It has also been running Buster (the one that will not boot, for a week). Last week it also did not boot (similar/same problems), but it went away. Some how it booted.

Update: problem found, now I need to find a solution

From gparted live. fdisk -l shows

• Partition table type (Disklabel type): gpt
• root partition type: EFI System
• 3 or 4 blocks free at end of disk (for 2nd copy of gpt)

I got it to work

see https://unix.stackexchange.com/a/535029/4778

However it is a bit of a kludge: it warned that it was an unreliable solution. Since doing this I have noticed that grub-pc is installed, and not grub-efi… (this may be because I had to start the installer in BIOS mode. I don't know why the partition table got converted to GPT).

Update 2020-05-25: it has been working without fault. (maybe it is only unstable if I mess with partitions. I have no need to mess with partitions until next time I replace the OS.)

Hardware: Lenovo Yoga 3-11 (the yoga3 with intel graphics)