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Which is the best application to Sync two folders?

I have been trying get my machine (XP-Pro SP3) to sync files to a local USB drive in the same way as I can with mapped network drives.

I particularly want to the sync to happen automatically when the USB is connected - in the same way that Off-line files will sync when the network drive comes on line.

I can get a folder on the USB mounted as a network drive, but cannot get XP to offer the off-line files functionality.

I have tried MS's SyncToy - it works as advertised, and will do scheduled and ad-hoc sync's, but does not seem to offer automatic syncs.

I suspect I could do this by plugging my USB into another machine on my network - but that seems heavy handed.

All insight appreciated - If you know this cannot be done please say so.