ADMIN: repairing efi boot on Debian

If you swapped hard drives, installed an UEFI OS on the new one and put the old one back, changes are you won't be able to boot. To repair that (on Debian at least), boot on a liveusb (or CD if you live in the past), mount your boot partition and do:

grub-install --efi-directory /boot/efi

Reboot and you should be on your old OS.