I'll applaud you for having a variety of drives and formats available just-in-case (rather than the ballsy "Everything is on this ONE USB drive, and it WILL work!" attitude). ;)
With some specific models, there could certainly be a conflict with the chipset manager (as is the case with the Corsair Survivor series drives in XDJ players), and that may be the case with your Ultra Fit in HFS+... but without testing, it's hard to say. It's unusual that they'd change their behaviour, unless they're just starting to show signs of failure. You might try formatting and re-exporting to see if that helps.
I typically have mine formatted HFS+ and haven't had issues with loading those in NXS2's ... but then again, I'm also using the best drives I can buy. lol