I say “finally”, there used to be a number of them. Back when I shot a pair of D100 bodies back in 2002, I used a 20GB Super Digibin 2 to backup my massive 128MB CompactFlash cards while still out shooting. It was slow and the battery didn’t last very long, but it was reliable and it worked. It allowed me to backup or unload my cards before I got anywhere close to home.
In recent years, such inexpensive and readily available backup solutions seem to have disappeared. There are a few out there, but many are either way out of most budgets, offer very little in the way of user experience, or just aren’t very good. New startup DFi Gear is looking to change that with the Flash Porter. It’s basically a hard drive (or SSD) in a small case with CF, SD and microSD card readers and a great big LCD screen to back up while you’re out and about.