okay, two things here. Firstly, will your BIOS boot from your external USB drive? If not, it may be pretty tricky to actually boot the system.
As for anaconda recognizing your USB drive, I'm not entirely sure, having not had much experience with anaconda. I do know one thing about USB drives, though, and that is in slackware, USB drives are recognized by using SCSI emulation....so it is /dev/sda1.