Include log to get proper answer on first issue.
About second - never defrag a drive that is in use.
If there is no programs using a drive, then it can be safely defragged.
Problems might come when using p2p program that is not following rules on disk usage. With those programs you might get all kinds of problems, and better option is not to use those at all.
Defragging usually improves diskspeed and filehandling, so it is good to do it quite often.