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.
I am trying to transfer numerous quicktime files from one WD hard drive to another and most go through but on some of them, I get an error message -36, which tells me that some of the data could not be read or written. However, I can copy the file on to the desktop no problem. Is the problem with the file or the harddrive? Is there a way to fix it? Is this a file corruption problem?