Does anyone have a solution for syncing large directories/filesystems using just a USB flash drive (and specifically without using a network connection)?

The objective is simply to sync a user directory between two computers. The contents of the user directory could amount to a large quantity of data—say, a quantity larger than could be stored on any single USB drive—but the aggregate size of changes that must be propagated by a single sync could easily fit on a USB drive.

As an example, suppose a user directory is already synchronized between a desktop and a laptop computer. Here's a use case:

1. Some changes are made in the user directory on the desktop.
2. We mount a USB drive onto the desktop and copy whatever changes need to be applied to the laptop user directory in order to synchronize the desktop and laptop user directories.
3. We now mount the USB drive onto the laptop and apply the changes.
4. The desktop and laptop user directories are now synchronized.

Any ideas?

Alan