Gosh Monty you may have a quota on a directory that is holding you back. Check and remove any quotas that may be applied to the source directory.  This problem happens frequently when you move files inside of the Isilon directory structure.  Always check and maintain your quotas you silly goose.
Having problems moving files from one directory to another. What could the problem be?
#mv ./olddir filename1 ../newdir/filename2 not working