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What is the best way to keep a folder synchronized with my USB drive?
I'm looking for a particular software for backup/synchronize.
I already ask to many people, but no one given me an answer, so I decided to provide an example of use.
I have two folders, the first on the computer's hard disk and the second on an USB Stick (or any other external drive).
The backup software have to do something like in this diagram:

foo and bar are the main folders; A, B, C, D are sub-folders; the others are just files.
Exist a program to do this?