I have two disks, with Ubuntu installed on both, and the boot menu displays both Ubuntu installations.
Ubuntu was installed on the second disk after the first disk, so I gather the grub menu is derived from that installation.
So two scenarios: 1) If the first disk fails so that OS1 is no longer available - will I still be able to boot OS2? 2) If the second disk fails (from where the grub menu is derived?) will I still be able to boot to the OS1?
If not, then is there a way (without using a live USB) to recover the other OS (will grub prompt be shown, or will it not boot at all? I am not sure how it works with EFI, I suppose in the past if SATA0 fails, then SATA1 will be the default?).
Note, the grub.cfg files on each hard disk include menu entries for both OS's.