When you say 'external' I take it is USB drive.
I have couple of questions for you:

Were you there when defragmentation took place?
What defragmenter was used?
Was it interuppted?
Was it on the same (your) machine?
If not did you try to plugg it in the machine that defragmnted it?
Does your machine support USB 2.0?
Did you try scandisk?

If you didn't do anything with the drive after defragmentation I suggest you do not try scandisk until all other possibilites were tried.