Best practice would be to leave Poser itself installed on your internal drive and create an external Poser library (Runtime) on the external drive. All your content (current and future) will be on the external drive. (I think there are some people who do have Poser itself installed on a USB or Firewire drive, but I would not do this myself).

I have only ever used external libraries, but If I were doing this I think I would:

1. Copy my Poser runtime onto the external drive
2. Link Poser to it as an external library
3. Go through the default Poser Runtime on the internal drive, deleting all my purchased content to free up disk space, making sure not to delete any Poser program folders.
4. In the external Runtime, delete the Poser program folders, such as CG, Dots, LipSync, Plugins, poserHtml, prefs, ui, and Weblinks, leaving just the content folders.

By the way, I would also recommend getting a two-disk external drive with mirrored disks. It's a bit more money, but using an external disk gives you this option, and think of what you would lose if your single external drive failed... which they sometimes do!

EDIT: WAIT FOR MORE RESPONSES BEFORE DOING ANYTHING!! Others may have better ideas than mine. (It's actually more than likely!)

"If I were a shadow, I know I wouldn't like to be half of what I should be."
Mr Otsuka, the old black tomcat in Kafka on the Shore (Haruki Murakami)