I'm with hfg on this, and am using a Gen 1 LaCie Rugged SSD right now (over TB) with a Sierra partition. I've not had any issues with Win 7/8.1/10 or Mac partitions over USB 3 for storage but for OS-related partitions I can't offer that USB 3 doesn't cause me problems more often than I want - I've had zero/0/nada problems with the same drives over TB 1/2...