Yes, you can use USB 2.0 devices. they report to the VM as 1.1 but run at near 2.0 speed. Some devices work and some don't.
You can use a USB mouse. OSX captures the mouse, and Parallels passes mouse movement, clicks (up to three switches) and wheel movement to the VM.
You can use an external hard disk to hold a VM .hdd file (or a whole VM), but you can't access it directly except via shared folders or the network (both techniques work).