If you have room on your C: drive you can run it from there. Then you can attach to your Library (Runtime) on your external drive. Shout if you need guidance on how to do that.

You don't have to re-install Poser itself on the external drive, unless you really are low on space and want to reclaim some.

"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)