Welcome to LQ! Your ability to boot from an external USB drive depends on whether your BIOS supports that option. Reboot, press the Del key to enter BIOS, then look at the options under the boot sequence. If you see USB, move it up to the top, and you should be good. If there is no USB booting option, then practically speaking you won't be able to boot from that external drive.
Frankly I'm a little puzzled as to how your Dell functions without any hard drive. If you post more details about your equipment, it will be easier for people to offer suggestions