I have two external drives with nearly identical contents, and I found that defraggler (an older version and the latest) crashed partway through defragging one drive, but was fine on the other. I couldn't work this out at first, even telling Defraggler to generate the most detailed log file didn't appear (to my untrained eye) to reveal any obvious cause -- the end of the file contained similar lines that occurred many times throughout the file.
Sometime later when I clearing up files on the drive, I selected some files in Explorer and tried to delete them, but Windows said it couldn't find them. I realised these had come from when I backed up a linux machine, and the filenames were legal on there, but were not legal on Windows (containing :, <, >, and the like).
It seems that this was the cause, because having found and deleted all these files (using linux) the defrag completed successfully. Would it be possible to stop this crash? I'm not sure how high a priority it would be to fix it (it can't be *that* common a situation, but it also probably shouldn't crash the program -- having said that, I've no idea how it should handle the situation), and it might be worth noting that the mere presence of such files doesn't cause a problem (the second drive also contained them, but Defraggler presumably didn't try to move them as it completed successfully).
I had a quick search of the forum, but this doesn't seem to have come up before. Sorry if I've missed anything.
Thanks!