Desktop Strangeness - no mounted volumes

Hi

Really puzzled by recent desktop behaviour.
Running Ubuntu Mate 18.04 LTS.

Was experiencing desktop slowdowns - was unable to copy paste or delete files or folders.
Also double clicking a folder sometimes would not open it and if it did the contents weren't visible, can only see spinning icon.
CPU process are normal, ie close to zero.

I rebooted and other strangeness happened.
None of my volumes are mounting on the desktop even though they are configured to using Mate tweak (attached)
Browsers and apps seem to launch ok.

Also if I go to Places in the menu all of my volumes open except for just one, WORK_1TB (attached link)

If I use Disks utility to check the filesystem on this external drive it suggests to repair but also warns that repairing may corrupt files. (attached link) This is my primary production volume. Before I attempt a repair I'd like to first back up important files but if I use GrSync I can see the volume but it won't open. Argh.

Can any Ubuntu wizards help me figure out what is happening?
Any feedback appreciated.