I have a late 2015 iMac Retina. I have a number of USB-3 external drives (with and without separate power), and none of them will mount in 3 of the 4 USB-3 ports on the back if the iMac. They all mount fine in the left-most port (near the Thunderbolt ports). And they all mount fine when attached to the iMac by a USB-2 extension cable and the iMaccompanion USB extension, which claims to be USB-3. The same holds true for my USB-3 thumb drive. USB-2 devices are working fine. Any idea what could be causing this? I'm running the 10.11.4 beta, but am not sure if I experienced this behavior before upgrading. Thanks.