|Why aren't you asking in the Linux forum?|
"I want to install Linux Freespire onto my WD external drive"
Of all the Linux distros available, why would you choose FreeSpire? The majority of Linux distros come on a Live CD...that means you can boot off it & simply run the OS directly from the disc without installing it.
Your external HDD may or may not be recognized when you boot off the disc...what you *may* have to do is boot into FreeSpire from the Live CD, then mount the external HDD so that it's recognized by the OS, then install the the OS from within the Live CD.
Or you could try following the installation instructions: