Like others I use a WD Passport USB 3.0 Drive (2TBs). I like that it can be easily encrypted and that it can be powered by the USB bus alone. Very small and portable as well, they even have a wireless model now that you can plug SD cards into (It's aimed at camera offloading). I have some thumb drives as well but for system images I like to break out the HDD.
I don't know the Drives RPM off the top of my head but the transfer speeds are pretty good, not SSD good of course. But it's usually the client's machine that is the limiting factor.