You could treat your photo archive's sudden status as a read-only disk, as an opportunity to buy a backup disk, and make a copy!
fsck is the repair tool you were looking for. First, some info. In a terminal window run
mount|grep media. It will print something like
/dev/sdb1 on /media/user/HD-drive type vfat (followed by some noise)
And maybe other disks as well. You're looking for the device ID corresponding to your mounted drive,
/dev/sdb1 in my example. Next, unmount the disk and run the repair tool.
sudo umount /media/user/HD-drive
sudo fsck.vfat -a -l /dev/sdb1
where /dev/sdb1 is the device discovered earlier (and probably will be not exactly the same on your system).
fsck will attempt to fix anything broken on your drive, and will list names of any files it found remarkable. Make a note of the file names, if any.
With any luck your disk will be fine now. Wait a minute after fsck completes, unplug it, and plug it back in. Though the disk (should) look healthy now, some files may be damaged. You may want to come back and ask for tips detecting damage, just for peace of mind. Be sure to check any files fsck may have mentioned.
Personally I would hie me off to get a backup disk, make a copy, and only then attempt the recovery process above. Any files you find were damaged, you may be able to copy back to the original disk from the pre-fix copy you just made. Unlikely but possible. Keep both disks then, and use the original for your working copy for a while, until you're pretty confident it's in good shape and the pre-fix copy is no longer helpful. Once you've reached that point, copy the original disk back over the copy, and begin using the copy as an active backup.