Why is my USB drive suddenly read-only and how can I make it read-write again?
Who's "Fetching..." ? 

I looks like it can't grab the correct perms/ACL. Have you tried setting up the perms again?
My guess is 'fetching' ought to be the _myname account, which my less than stunning grasp of nix perms leads me to believe is the system process working in your name.

ie it's lost the permission to work on your behalf.

At minimum, I'd guess setting everybody to read/write would at least be a workaround - as there's no ownership, how does it know it's working on your behalf or anyone else's?

[Someone with a better grasp of nix perms please comment/correct/append to this]