Well, maybe I didn't understand you correctly. What do you mean saying a USB drive? Is it a USB flash memory stick or a CD drive that connects to the computer through an USB port?
If the first, when it's connected to the computer, it appears in the list of startup devices in R-Drive Image, and you may create a usb rescue disk out of it.
If the second, a USB cd drive should appear as a regular CD drive in your computer. If it can write discs, it'll appear among the list of the devices, and will be able to create a rescue CD disc. If not, you may write an ISO image and then create the rescue disc using some other CD burner.
The rest depends solely on your computer - whether it can be booted from an external USB flash memory or CD drive. Maybe, you'll have to set something in BIOS.