Remember that with at least some USB3.0 ports like in RB3011UiAS a USB 3.0 drive will not be detected! Even rebooting with it inserted desn’t help. It seems to have to be USB 2.0 drive for Mikrotik to work.
I solved this (or worked around) by connecting my USB 3.0 pendrive through USB 2.0 hub to RB3011UiAS, and it worked immediately 
But for hAP ac2 this is no problem at all, as it detects same USB 3.0 pendrive directly.
Some guy had similar issue:
http://forum.mikrotik.com/t/solved-usb-flash-not-showing-up/102863/1