BecomingJulie
Bluelight Crew
Actually, you probably could install Linux on a USB external HDD, modern motherboards should be able to boot from one. I was just thinking of a naked drive. Screw it into a bay just above or below the existing one, and swap over the power and SATA cables (it probably will be OK to leave power cables plugged into both drives and just swap the SATA cable. But if something doesn't seem to be working, first try depowering the unused drive.)