Late 2013 iMac i7 3.5GHz. I created a clone on an external Seagate UBS-3 drive. The cloned external drive will not boot using a USB-3 cable, but it will using a USB-2 cable. The drive works fine as a USB-3 drive if I boot from the internal drive. Is there an incompatibility between these iMac models and external USB-e drives, as far as a boot drive goes? Thank you!