dump -0uf /media/usbdisk/makes a full backup of the file system into the specified file (in this case mounted on /media/usbdisk), and tests it afterwards (that's what restore -C does). In my case, a roughly 6.5G backup took 12 minutes to complete (over USB2.0) - I didn't keep track of the test time.
restore -C -f /media/usbdisk/
Of course, for single-file or -directory backups, you need something else. I haven't completely settled on a structural solution, at the moment I'm using a mix of rsync (to save to a network drive) and bzr (to keep revisions of my work without need for a server).
With thanks to our very knowledgeable Computer Officer at work for pointing me to dump.