Try this
Take the hard drive out of the external shell. (there should be a couple screws on one end, at least one should be under a warranty sticker). Then simply change the jumper setting to slave and install the drive into the pc. I did this just last night after having the same "post-defrag" problem and now it works just fine. The only catch is that I had to format the drive as it was now recognized as a new drive, and it was partitioned at fat32 and i'm using ntfs. Also if you can get a copy of Partition Magic it will format and partition the drive a lot faster that letting windows do it.
I'm going to try putting it back into the external shell and see if it works again. (i'll update on here with those results)The only loss was i had to say bye bye to all the data on the drive, but it works. =)
I hope this helps anyone else with the problem.