On Mon, 9 Mar 2020 at 11:06, Chris Green cl@isbd.net wrote:
Personalities : [linear] [raid0] [raid1] md4 : active raid1 sda4[0] 973522816 blocks [2/1] [U_]
[...]
Which led me to the answer (eventually), I was seeing the error (on the GUI) 'Failed to mount /dev/md4' but from the above md4 seemed to be OK. On looking in /dev I saw that /dev/md4 didn't exist, [...]
Very odd, I don't see how mdstat could have been telling you that md4 was active (albeit missing one of its components) without md4 existing, but I'm glad it led you to a solution!