[Hampshire] Peculiar behaviour of USB disk

For years I have used an old 40Gb USB disk for backups. My photograph
collection grows ever larger (as they do) so I've acquired a Western
Digital 250GB device. All is well, most of the time, but I've noticed
something odd. When I try to copy a particular directory with 104
files from hda to the new disk, it copies 33 of them and then stops.
When I try again, it again stops after transferring 33 files. There's
another one which stops, every time, after transferring 17 out of 49
files. There are 35 directories in all, larger and smaller, and every
other one has been copied satisfactorily.

Any ideas? System is a ThinkPad A21m with Dapper Drake.

CAS