It may be that your registry is not recognizing the external drive.  I also run poser on a separate drive, but it is one attached to my home network.  

This may sound crazy, but this is how I got my Poser to play nice with my lap top and my tower.  I ran the install program on both machines separately, telling it to load onto my "X" drive (the external).   This sets up the registry files to know where I want it to look for the program.     

I ran the setup on the second machine telling it to land in the same place (yes it over wrote the first installation on the external drive,  but since I hadn't used it yet, there was nothing to be lost, and it was transparent to the first machine.)   

This way, I have all the registry files where they belong on either machine, and the program and runtimes look to the external.    The only problem I have is if the external drive gets turned off.  

I hope this wasn't confusing.