I've used this freeware program called Good Sync on my laptop to sync the "My Documents" and my email folders to my 512M USB key. Sure, it's not perfect, but it's close enough.
I came across Good Sync a few months ago and downloaded version 3.something. I setup a job for each folder and I scheduled it to run every 30 minutes while the USB key was attached.
Today I got a message saying that version 4.6.1 was ready and asked if I wanted to update. I thought "Sure, why not?". I downloaded the software, tried to manually sync up and got an error saying my jobs were too big for the free version of Good Sync and I'd have to pay $19.99 to purchase the "Pro" version. Nice.