Holmes, I had the same problem and was told that I installed a program with my external on. What I was told and done was to use a recovery program and recover my files as my external was full of music, pictures and films. I had to buy another external as I had not enough room on my computor for the amount of info on the external, then reformat the external and replace it all again.
If you do not have a lot on the external and your pc has enough room to store it all, recover it to pc. If not enough room on pc then recover to another external, pc, laptop etc.
DO NOT reformat until you recover your your stuff.
hope this helps.
let me know how you get on.