Booting from USB 3.x devices may be problematic given the implementation details of USB 3.x support on some boards. Let me explain in detail:
For example, on my Gigabyte GA-P67A-UD3-B3 there is a Renesas (NEC) USB 3.0 controller associated with 2 ports on the backplane. You cannot boot off of these ports, for what (to me anyway) is quite obvious: it's a third-party chip, the BIOS would need full emulation shims and a BIOS-level driver (code) to talk to the underlying Renesas controller and provide the necessary tie-in shims. It's a lot of work, and not feasible. These chips do not provide xHCI (keep reading) to my knowledge.
I also have a Renesas (NEC) USB 3.0 PCIe x1 controller (physical card) in my system too, which provides another 2 ports (this time wired to the front panel of my case). One cannot boot off of these ports either -- to do this, one would need an option ROM, which the card does not have. Furthermore, option ROMs often do not "stack" well. (Example: a system with AHCI enabled + a SCSI controller + PXE network boot all enabled probably won't work, but it depends completely/entirely on the motherboard) These cards also do not provide xHCI.
Expanding even further: the newest/latest Intel chipset boards (e.g. Z77, etc.) provide native USB 3.x support, except the implementation there is something I haven't even begun to read up on yet. The newest fun thing is called xHCI
(not to be confused with OHCI, UHCI, and EHCI -- all of which are described here
), and systems that use xHCI
provide a multitude (literally 6 or 7 options; download the User Manual and see the BIOS section) pertaining to booting xHCI devices (whether they be USB 3.0, 2.0, 1.1, or 1.0).
Thus, with xHCI, USB 3.0 devices will use xHCI, USB 2.0 devices will use the EHCI capability (despite attached to an xHCI controller), and USB 1.1 and 1.0 will use UHCI. This is exactly how the existing USB 2.0 model works, and how/why the USB ports are all "universal" in capability. I will note that there were some early USB 2.0 adopter motherboards which had separate USB 2.0 and USB 1.1/1.0 capable ports, which was very strange but did exist. This is not the norm these days; a USB port is a USB port.
The downside to xHCI is that Intel, for whatever reason, is refusing to provide Windows XP drivers for it, citing that "it only works on Windows 7", and as such you can only achieve up to USB 2.0 speeds with xHCI controllers on Windows XP. This affects people like me greatly since I refuse to go to Windows 7 for reasons I also refuse to get into here.
My point is that the reason you can't boot from your USB 3.0 devices is probably because of my 2nd paragraph above. That has nothing to do with "device compatibility" (specifically USB 2.0 devices not working on USB 3.0 ports), it has to do with how PC architecture is poorly designed. Then comes the whole UEFI/EFI thing which I don't want to get into either...
Footnote: isn't it great how USB stands for Universal Serial Bus, except there's 1. nothing universal about it (nobody seems to think ahead when designing these protocols), and 2. absolutely no USB standard/classification for serial ports? Welcome to what committees do these days -- very little.--
Making life hard for others since 1977.
I speak for myself and not my employer/affiliates of my employer.