I described creating an Encrypted USB Key Fob previously. Here is a script I use to backup my data to it. It rsyncs a few directories and will also download your calendar and contacts from Zimbra if you are so blessed.
Just put this script on the key in bin/backup. Put the key in each computer and cd to /mnt/disk/bin and run the script. If you are not prompted for the password when you insert the key run 'sudo /sbin/cryptsetup luksOpen /dev/sdb1 dm-usb'.