My apologies for not mentioning, that my successful use of the Patriot Magnum flash drive on the AC68U USB 3.0 port was with the flash drive Linux-formatted with ext2. My way of doing the ext2 formatting was with a freeware Windows utility.
Turns out mine is formatted exFAT. That must be my problem, as Merlin above mentioned that exFat is not supported. I've got a 32 GB Patriot Supersonic (also USB 3.0) that's formatted FAT32 - I might give that a try instead.