Am 07.02.20 um 15:49 schrieb Ram Ramesh:

quoted

quoted

4. Make both bootable. (I suppose I need to change UUID of md1
    partitions. I suppose that is easy)

Yes it's easy. Yes it should have been done a LOOONG time ago.

Not if you break md0 into degraded md0 and md1. grub.cfg need to be
updated after that fact and UUID update is only doable after breakup.
Again not sure if I am missing something important implied by LOONG in
your answer

grub itself is installed before the RAID and there is no need to change
any UUID - you just spit out one of the disks from your RAID, work in
degraded mode and if everyhting goes wrong you boot with a recue media
and assemble that disk, zero the one with the damaged OS and resync