You can have runtimes anywhere.  I have runtimes on three separate hard drives, with poser running on my main C drive.  And, yes, you probably can run Poser from any drive, too, though if it's not a serial drive, it will slow down considerably.  I suggest keeping Poser's main install on your C drive, and using external runtimes, just as IsaoShi suggests.  Just remember a few things:

1. DAZ main figures (V4, M4, mil2dragon, etc.) don't like to be moved from your main runtime.  I suggest leaving them there, unless you go to the DAZ threads and read EXACTLY how to install them in other runtimes.

2) Python scripts must be installed in your main runtime.

3. If you boot poser with your external drive unplugged, it will forget about your external runtimes, and you'll have to re-add them.

4. Take IsaoShi's advice and get a redundant, self-backing drive. I lost ALL my runtimes about two months ago and had to start over.  NOT fun, I assure you.  Thankfully, I had backups of most of my install files on another drive.  In any event, back up EARLY and OFTEN.

Good luck!

Intel Core i7-8700 6-Core 3.2 GHz (4.6 GHz Turbo), 32GB RAM, two GeForce GTX 1080 Ti GPUs

DS 4.10, Photoshop CC and CS6, Poser 11 Pro, Vue 2016, CarraraPro 64bit, Autodesk Inventor, Mudbox, and 3DS Max

One in 68 children is diagnosed with autism.  One is mine. ♥