Success! So I have the server back up and it looks to be working as it was prior to the "issue" with one exception. I have errors on my 2nd cache drive (refer attached image). I think that drive may be on its way out??? I better look at how I can undertake some diagnostics on the drive to get a definitive answer as the drive is currently still under warranty.
Another thing I noticed was a new section on the Main tab called Historical Devices (refer attached image). Does anyone know what this is and should I just delete these OR do I need to do something with this detail?
I thought I'd quickly jot down the main steps I had to go through. Hopefully it might be useful to someone else?
Mount the drive that is reporting as "Unmountable" (using the FAQ referenced in my previous post).
Backup all data on the drive to another drive (I used rsync. e.g. rsync -aczvAXHS --progress /source /target Note: I found that the source directory was created in the target directory with all of it files and folder structure).
Compare the backed up data with the original [optional] (I used rsync. e.g. rsync -n -avrc /source/ /target/source/).
Unmount the drive (e.g. umount /source).
Destroy data on the drive (my device was a SSD so I used blkdiscard /dev/sdX).
Format the drive via the unRAID GUI.
Copy all data back onto the cache drive (rsync -aczvAXHS --progress /target/source/ /mnt/cache).
Compare the restored files [optional] (rsync -n -avrc /target/source/ /mnt/cache/).
Reboot the unRAID server.
techscholar's post in Mirrored Cache Pool Showing 'Unmountable: No File System' was marked as the answer