What steps will reproduce the problem?
1. I connect my mp3 player (Sansa Clip 8Gb) via USB
2. I set up my preferences
3. I click 'Synchronise'

What is the expected output? What do you see instead?
I expect the files on the mp3 player to sync with my selected playlist; for
iTMW to delete files from the player that are not in the playlist and add
files from the playlist that are not on the player.

My first sync takes a long time, but works. The player is filled with my
tracks so I think the preferences were set up OK.

But each time I go back to sync again, I get the message saying the plylist
will be copied to the mp3 player folder and that all the files in that
folder will be deleted. It then deletes ALL the files and re-copies them -
taking hours each time.

What version of the product are you using? On what operating system?
I am using iTMW 0.951 and iTunes 8.0.2 (20) on Mac OSX 10.4.11

Please provide any additional information below.