Usually the installation process has an option to start a command shell to let you snoop around the system.
Then if you scroll through the output of the 'dmesg' command (run 'dmesg | less', and hit the spacebar to scroll) you should see messages about the USB drive getting configured and its device name (/dev/sdb or whatever).
Otherwise you could try disabling the built-in HD in the BIOS before booting the linux installer. I'm not completely sure this will stop it showing up as an option.
You may have to edit the boot configuration (in file /etc/lilo or the GRUB equivalent) to get the disk to boot back in the old PC. Not sure.
Last edited by maroonbaboon; 08-29-2007 at 11:19 PM.