Licensing on the software I am using is OK with uninstalling it from one workstation and reinstalling on another. Installing it on a hard drive on each workstation would violate the license. I think that what I am suggesting is in the area in between those two cases, and I would contact the software manufacturer about it if the Windows O/S would deal with it OK. Their machine key can be set up to look explicitly at the hard drive and no other hardware, so it would support this usage.
To deal with different hardware configurations, I suppose that the portable drive could be partitioned with a windows installation for each. It seems like that would violate the Windows licensing though.