The label on the missing drive (in your first post) didn't look right, so I set up a VM to look at OMV's naming convention.
Note the differences between the device names in the following. The first line is a single drive where the second line is a RAID1 array. /dev/md0 or /dev/md127 are typical device names, assigned by OMV, to a RAID array when it's created.
I killed one drive in the RAID1 array. Nothing changed in the file systems page (above), but the array's status is now degraded in Storage, RAID management.
Then I killed the last drive, of two, in the RAID1 array. Note the Device Name change of the array, in the file systems page.
It went from /dev/md0 to; "md-name"+ the host name + the original label. (At this point, there's nothing shown in Storage, RAID Management.)
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So this leads me to ask, is the missing device really a single disk that happens to be named "RAID1" or do you have a missing RAID1 array?
This is more of a curiosity item than anything else. It doesn't really change anything in that proceeding with a fix or repair, from here, would likely require getting on the command line.