My 13" MBP retina, fully updated as of today is and has never been able to boot from USB. I'm talking both Linux and OS X USB drives which are no problem to boot in other machines, including some older Macs. I've tried refind without any luck. The computer simply doesn't seem to see any USB drives until booted. No problems using USB 2 / 3 devices in ML. I've read about people booting Linux on these Retinas using normal dd of the image to a USB key so what gives? Am I doing something wrong or some new machines just unable to boot USB? *I should mention the computer does boot from the SD slot.