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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.

Thanks

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    With Ubuntu's grub you boot only from the one ESP - efi system partition on drive seen as sda. I create an ESP on sdb and copy all of sda's ESP to sdb, but those boot files would not automatically work, but would make it easier to repair. If your configuration has ESP on one drive but install on second drive, both drives must work to boot. I like to keep a current version live installer handy for every system I have installed. If you have a /EFI/Boot/bootx64.efi in every ESP, you may be able to directly boot from UEFI. – oldfred Mar 14 '18 at 23:00
  • Thanks - when I installed OS2, it initially booted only the second OS and not the first, so it seems it is booting from sdb ESP, is this possible? – sumade Mar 15 '18 at 0:04
  • If I add a manual entry to sda grub.cfg (40_custom) and update-grub so both OS are in the boot menu, then can the ESP (/efi) be trivially copied to sdb /efi, such that sda and sdb will both have bootx64.efi? (given this, will it be possible to "directly boot from UEFI"? - if i remove one or the other of the disks can I effectively simulate a failed disk to see if it's possible? - the problem is that I won't be able to use live USB remotely, but can access reboot/enter BIOS etc.), in the unlikely event that the first disk fails or something otherwise goes wrong, I need the second OS – sumade Mar 15 '18 at 0:06
  • When you use shimx64.efi from a full install, it expects more files from /EFI/ubuntu, so you need both /EFI/Boot and /EFI/ubuntu in both ESP. And then the /EFI/ubuntu/grub.cfg needs to have configfile settings for correct version. askubuntu.com/questions/1002789/… & example: askubuntu.com/questions/958760/… – oldfred Mar 15 '18 at 3:27

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