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Boot problems are really tricky to fix on the computer that's having them.
If any of your other OS's still boot, then use them to download a live ISO of your preferred linux distro, and write it to a USB stick.
Then you can diagnose the problem using a new boot environment that doesn't rely on the main disk.
Thanks. I haven't really tried to fix it very hard, but that's an obvious step I should have tried.
If the Debians you installed weren't Live ISOs, then I recommend Arch or Artix for CLI, and Manjaro or Ubuntu for GUI.
You'll have to mount the drives, chroot into the one on the drive that has the EFI partition, then grub-install and grub-mkconfig into the EFI partition.
Thank you, I shall try it.
I'm thinking you may have updated grub at some stage from Deb, and didn't have the test OS mounted at the time, or os-prober not enabled :- therefore not detected when grub.cfg was regenerated.
https://wiki.archlinux.org/title/GRUB#Detecting_other_operating_systems
https://wiki.debian.org/Grub#Dual_Boot
I ended up using rEFInd myself, as it does automatic OS detection/scans for bootable partitions.
I didn't update GRUB intentionally, but who knows. Installing and reading docs, thanks for the tip.
I can get into initramfs on that drive now, but the first thing it says is "no root device specified. Boot arguments must include a root= parameter". I assume I'd add that to GRUB, if I could see the OS in GRUB!
Then from the BusyBox initramfs prompt, i try vgchange -ay to try and activate the LVM partitions....? , and it activates three logical volumes from each of the two Debians. I figured out that I can pass it my correct LVM group, and it only activated those three. I exit from the initramfs Prompt and it says it can't find some files in /root/proc, then it hangs forever.
Man, all the stack overflow stuff I see says things like "mount the root to a new folder, then fix the problem, then reboot." But doesn't ever explain how to actually do some of it. I'm just too new to follow the "yada yada"!