Quote - "This time around we are going to put the runtime folder on an external hard drive, and I have some questions before I mess that up."

 "Also, is there a way to stop the external hard drive name letters from changing?"

"Anything else I should be aware of with external runtime?"

Go to this link to read about External Runtimes and you can download the actual folder structure that I use in my exteral runtimes. It'll save you some time from creating folders.  I use multiple runtimes for better management.  I explain how to edit the folders and how to install content in them.  I'll actually be demonstrating this in my online workshop this week.

As for your computer drives changing drive letters, I would recommend turning the external drive, that keeps changing drive letters, on first.  This will ensure that the drive letter will not change for that drive.  I get the same problem from time to time and I have had it happen on more than one computer. If I had to guess the cause of the problem, then I would probably say that it's likely to happen when the external is turned off mannually while the computer is on and not after the computer shutdown.