Files copied to a disk that is improperly unmounted or disconnected may not appear on the disk when it is connected again. For example, if you disconnect an external FireWire-based hard disk from your computer without first unmounting the volume in the Finder or Disk Utility, this issue may occur.
Properly unmount the volume before disconnecting the disk.
If this issue occurs, you should use Disk Utility to verify that the volume is free of directory damage. Repair the volume if any directory issues are detected. Then, copy the data to the volume again (and unmount it correctly).
This can happen if the disk cache is not emptied ("flushed") due to an improper unmount of the destination volume.