I have more or less come up against this issue as well, with one exception;
two of the USB 3 ports on my Asrock FM2 AMD mobo are powered when the PC is off, i.e. to charge devices when the PC is not running. Ubuntu 14.04 LTS detects external USB3 HDD devices when they are plugged into these ports. The other four USB 3 ports are not detected.
I have more or less come up against this issue as well, with one exception;
two of the USB 3 ports on my Asrock FM2 AMD mobo are powered when the PC is off, i.e. to charge devices when the PC is not running. Ubuntu 14.04 LTS detects external USB3 HDD devices when they are plugged into these ports. The other four USB 3 ports are not detected.