Try checking the permissions on some of the files that fail to move. My external is used between my machines and some folders created on one Mac aren't always allowed to be accessed by another. Check Disk Utility if getting info on the files doesn't tell you anything.

Can the files be opened? Are they corrupted? Maybe your HD is the one at fault, so perhaps storage is a problem, though I doubt it since it would tell you. I still think its a permissions issue.
-BMF