Hmm had something a bit similar to this happen before. My problem didnt have anything to do with defrag but I kept getting the "Delayed write fail" message (similar to what your getting) but I couldnt find anything physically wrong with the drive (ie benchmarks etc ran fine).
AFAIK I know it was caused because I was actually trying to copy some corrupted files to this disk. Once i got rid of the corrupted files the disk worked fine and has done so ever since.I dunno if this helps but you could try benchmarks etc. I presume you can still read all the data on the disk?