Cannot copy file <x>: parameter is incorrect

XP SP2, MSI motherboard, rather new machine.

I tried to drag and drop folders with mp3-files to my wifes new Cowon U3 mp3-player (both with Windows Explorer and with the Jetshell program from Cowon). Using the same backside-USB as I use for a removable hard disk (for backup) and a Sandisk USB-stick without any problems.
This however, was utterly unreliable, and unworkable: some files flashed by in a second, most took a minute and occasionally I got the above error. No obvious relation with file contents, as the same file could be transferred in a second or third attempt.

Googled the error, and found some hits, but not help at all. Either they only mentioned the problem without any solution at all (just 'help, I'm getting crazy' or they were
- not applicable (copying a file > 2Gb or > 4 Gb to a FAT32-partition)
- CD-specific issues (like a too long filename or CD not closed)
- not helping (check the drive for bad sectors, I did a chkdsk /r)
- plain nonsense (retype the command)
- about a SD-card
- incomprehensible to me: http://support.microsoft.com/kb/832430
- anecdotic ("t didn't work in the shop either, but then he tried in on a demo computer and that was OK")

Before giving up (or getting crazy) I decided to try the front-side USB. And to my surprise that worked flawlessly, transferring whole folders without any error and any delay.

I'll leave it here, I think. Don't want to try things like:
- return the player
- reset the BIOS
- update the BIOS
- update motherboard drivers
- reinstall XP
- replace the motherboard
when there is a perfect work-around: just using another USB-connection.

But I would appreciate if someone could tell me what the cause could be. It's not a really satisfactory solution for me at the moment.

Kees