Advanced Flash Synchronizer 1.7 will help you back up and synchronize data on two computers that is not connected directly by the help of the external Flash/USB/ZIP drive.
The key feature of the AFS is copying only changed files to the external drive that allows you to keep synchronized a huge amount of data by using a small (128 MB for example) external drive.
The AFS stores the modification dates and sizes of all tracked files on both computers on the flash drive and copies only files that was changed since the last synchronization session.