Anything Goes
I know this isn't probably the answer you're looking for, but anything pretty much goes with externals. My brother is a closet Mac guy (meaning he still owns a PC but is almost ready to convert) and we have 4 external hard drives we use. They are all different and they all work for both computers. So, unlike many other things, external hard drives seem pretty versatile. I've never had any formatting problems with any of them (aided greatly by us both using VLC). Hope this helps!