Peter, we exclusively use USB2 (not USB3) flash drives for the MEP, and have never had a problem. We use 128GB Patriot Xporter Magnum drives. We format them with the HP Disk Storage Format Tool, v2.2.3 using Parallels/Win7 on a Mac. They work equally well on the front or back port of the MEP. We never use a hub or second USB device simultaneously.
This solution has been flawless for us.