File systems problem.

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.

Do you know how to do a screen shot of the File Systems window? I'm curious about what's actually missing. (I think in Win7, and up, there's a "Snipping" tool in accessories.)
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In any case, I can't see how this could be fixed without getting on the command line. That doesn't mean that you have to rebuild OMV but, you'd have to remove references to the missing drive and wipe / reformat the drive to use it again. Your call.
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If you decide to fully rebuild, take a bit of time and skim the New User Guide. There's a few utilities in it that will help you get a better look into your server, along with an example of how to setup Rsync backup.

OK, I ran a second test, this time in OMV4. (The prior test was done in OMV3.) When the array went missing, the device name remaining is /dev/disk/by-label/RAID1

So, the naming convention changed between OMV3 and OMV4.
Thanks for the screen shot.

Let's recap what we've learned.

The drive appears to be "functionally" OK - which means it can be reused. (Crystal Mark)
The drive appears to be 1 drive in a RAID array. This might be a good thing, depending on the answer to the next question.

In Storage, RAID Management, is there an array visible?
If there's something there, post a screen shot. You're looking for something like the screen shot of RAID Management that I posted above with a status of; clean, degraded.

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If there's nothing there and without getting on the command line, you'll be starting over.

You may not have to rebuild OMV, but you'd have to remove the references to the missing drive. Frankly, if it was me, I'd start from scratch. A rebuild takes 15 minutes or so and this route might be faster than trying to clean up references to the old drive.

This time around, take a look at the New User Guide and consider using Rsync in place of a RAID array.

Since you have two disks; if you'd have had an Rsync'ed backup disk, you'd still be running.

Regrets.

And (since you're a Brit), Happy Brexit!!