If you're wanting to move the whole program over there, I'm not sure that will work very well for you. Windows gets kind of persnickety about that. However, what I do is copy/move my runtimes to it so the content doesn't clog up my internal hard drive. It's also where I save my renders, etc.

Create a directory and call it anything you want. I called mine Poser 7 just for ease.
Using Windows Explorer or My Computer, locate the poser directory on your hard drive (C:Program Filesefrontierposer n)
Go down one more level to runtime. Select it, right click your mouse and choose copy
Navigate to your external drive (E: F: G: whatever)
Right click your mouse and choose save

You can then delete some of the stuff in the runtime folders on your internal hard drive, but I'd advise you to leave it just like it is. The only REALLY IMPORTANT thing to remember is that, when you're adding content and unzipping it, you have to navigate to the runtime folder on your external drive instead of the runtime folder on your internal drive.