The actual licensure would depend on the details of tghe specific language used in each license agreement and to some extent how rigorous the publisher is in enforcing the agreement. I can make two arguments..
  1. If the software is installed only on the portable external drive then it would be installed only on one computer at a time which I believe would meet the intent of the license agreement. However, I am confident any competent attorney could make me spend tens of thousands of dollars defending that position in court should the publisher so desire.
  2. I would look further in the license agreement to ascertain if there is a clause regarding installation of the software on a network. It strikes me that a portable drive moved from machine to machine could technically be considered a "sneaker" network and therefore not kosher.
In most cases I suspect number 1 would prevail.

If we knew what it was we were doing, it wouldn't be called research, would it?

— Albert Einstein