From what I have parsed the OP wants to have a full back-up on a USB-HDD and the diffs on the USB-Flash, because the Flash is limited in size.
Just write two rsync (or grsync) scenarios: one for HDD and the other for the Flash. On the HDD you will have a directory that is a mirror copy of your laptop. On the Flash you will keep the diffs for the time between syncs to the HDD.
When at home
1. rsync your laptop to the HDD (mirror).
2. copy the incremental stuff from the Flash to a separate directory (e.g. diff-2012-May-21) on the HDD, and wipe the Flash.
At the road:
just rsync diffs to that Flash.
I guess the recovery plan is quite obvious too. Should any _one_ of those three devices die, you are still good to go.