After expanding a raid 6 mdadm array from 4 to 5 drives, with --add and --grow, and before any updates to grub were made, computer was restarted.
At boot, it now says something about an invalid arch independent ELF magic, and a grub rescue> prompt.
Since the raid hosts /, I wonder if the ability of grub to assemble the md0 to get to /boot is now compromised. I have no idea how does that part work...
What I have done, I booted from the "live usb" and have also found that as a part of my previous attempts to boot from the raid, one drive fell out (sdc), and --assemble --scan was only using 3 drives, ending up in degraded array. I have probably accidentally unplugged the drive, when checking cables out of desperation , but in any case - I have plugged the drive back in, and it was still not willing to join, complaining about superblock.
Anyway, I have started the --manage --add, and it is resyncing now. When it is done, I am not sure what to do... I will attempt the restart, but something tells me it will fail again.
Do I somehow need to fix the grub record first?