You don't mention your OS but at the XP Command Prompt, or in a script, you could use DIR with the /TW parameter to display the date written (DIR PATH\FILENAME.EXT). I cannot find that the "date modified" is available. If you accept that the date written and date modified are the same then /TW will give you a start at what you want.
Then you could extract the date written for both files (on U and X drives) and compare them. If the date written on the source file is later than that of the destination file you could then copy.
Good luck.