You can, but external drives tend to be slower, so it might affect the speed in which Poser retrieves the content from inside your library.

I have my main drive partitioned and my my main runtime is in the default location on my C Drive, but I add very little to it.  The only thing I add to that Runtime is:

 

1.  Python Scripts because they don't run from an outside runtime;

2.  V4/M4 installs

3. !DAZ morph folder from my external runtimes.

 

Everything else gets installed into my external runtimes.

My external runtimes with all of my content is located on my D Drive (part of the main drive that was partitioned). So it's set aside, but not on an external hard drive.

Also, I'm learly about external hard drives. I figure that if they are constantly being accessed, they have to power up or whatever they do when they make that sound that they make when you go to access them in "MY Computer", that they might burn themselves out or something.

I do have a copy of my runtimes on my external drive for safe keeping. But as I said, I access them from my D Drive.

"It is good to see ourselves as others see us. Try as we may, we are never
able to know ourselves fully as we are, especially the evil side of us.
This we can do only if we are not angry with our critics but will take in good
heart whatever they might have to say." - Ghandi