3 computers: XP Pro, NT4, Win2000 Pro in a ethernet LAN (workgroup.)

Everything works fine for a long time, then suddenly:

There is a drive (a seperate physical drive) in the XP Pro comp. that the other 2 computers can read and copy data from but cannot write or copy to it.

The funky part is that:
the other 2 computers can write or copy to any folder in this drive without problem. Just can't write/copy to its root directory (anymore.)

Any idea?