will the original media files residing on the external HD be changed or edited in anyway or will they remain the same?
It is my understanding that these files will remain unchanged...
I would appreciate it if someone could confirm this to be true.
Premiere will not alter your original media files in any way.
When working from external HDD's, here's a tip: make sure to set the drive letter for that external (on a PC) HDD in the computer's OS, so that it will ALWAYS be the same, everytime that it is plugged in, or turned on, regardless of the order.
The links to those media files are absoute Paths, so if anything in the Path changes, the links will be broken.