Silly me, that's what you were asking about in the first place... innit?

On your Library palette in Poser, go the top folder level, where it shows your different Runtimes, click the + button down the bottom, and then select the PARENT folder of your Runtime folder on your external drive.

You will need to have that drive connected whenever you open a scene containing things from that Runtime, of course.

"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)