Can you boot a MacBook Air from a USB 3 external hard drive?
Solution 1:
Yes, I just got a new Mid-2012 MacBook Pro 13" and did this very thing over USB 3.0. I know of no reason why it would not work on a MacBook Air.
When booting from an external spinning hard drive over USB 3.0 on my new MacBook Pro, the external hard drive sends data back and forth to the Mac about as fast as it would if it were installed internally. I have not specifically measured drive speeds in this scenario; I'm just saying that it performs quite well.
As far as I know, all Macs with USB have been capable of booting from an external drive connected via USB--all the way back to the first iMac circa 1998, which only made use of USB 1.0. Booting from an external USB 2.0 drive, whether a flash/thumb drive or a spinning hard drive, is, however, quite slow.
When you boot your Mac, immediately after the startup chime, hold down the Option key. Your Mac will show you a row of icons of each volume or partition currently connected to the Mac (internally or externally) that holds a Mac OS X installation that you can successfully boot from. Use the arrow keys to select which one you want to boot from, and hit the Return key to boot from that volume.
Solution 2:
In some testing on a MacBook Pro with USB 3.0 and an earlier MacBook Pro with USB 2.0 (no USB 3.0) I've found
Both will boot to an external disk (a clone of the internal drive) if the drive is in a USB 2.0 case.
The USB 3.0 external case drive can be read by both machines. It does support USB 3.0 on the newer MacBook Pro.
The USB 3.0 drive is clearly an option in System Preferences, Startup Disk, and I can select the drive just fine.
The USB 3.0 drive will NOT boot on the newer MacBook (but it boots fine on the earlier MacBook with only USB 2.0). In fact, when booting holding down the option key, even then the external USB 3.0 drive is not visible on the newer MacBook, but is visible on earlier MacBook.
Once the OS is fully up and running, the USB 3.0 disk shows up fine as a device in Finder, it works at USB 3.0 speed, etc.
My Conclusion:
USB 3.0 is NOT supported by the internal firmware in the newest MacBook Pro so it cannot boot to the drive, but the Operating System itself clearly adds the support for the USB 3.0 device once it is loaded.
USB 3.0 is supported only in the operating system, and not in the firmware.
This makes the USB 3.0 support far less valuable, in my view, since booting to an external USB 3.0 device may be a very handy way to have a backup of the internal drive that can be used a reasonable speeds.
At this point, I have to put the external drive in a USB 2.0 enclosure. It boots, but offers absolutely no advantage on the newer MacBook with USB 3.0 support.
Perhaps we'll see an updated firmware that solves this problem.
My current Boot Rom Version is: MBP91.00D3.B08
I looked here: http://support.apple.com/downloads/#macnotebooks but didn't find a description of this USB 3.0 boot issue (yet).
Solution 3:
Various generations of Macs have different capability booting from different external drives. Before intel macs (2006), the external boot was generally supported on Firewire and not usb. Since "intel", USB is supported as well as firewire.
I am an experienced MacOSX user, and I have just experienced some issues with booting off of an external USB 3 drive. I suspect it has to do with power saving feature of external drive (that uses battery power) that counts on smarter power control than is currently done during the usb 3 booting process.
In particular, I experience trouble booting from a 1.5t seagate goflex USB 3.0 portable drive on my 2012 macbook pro 9.2 using Macosx 10.7.4. I think it is a driver / OSX version problem, but I haven't resolved the problem quite yet.
You have to have a "good" version of MacOSX that supports your model of Macintosh: There are subtle things that can happen among recent 10.6, 10.7 and 10.8 sub configurations that sort of customize themselves for specific models of intel macintosh models and loose some capabilities of running all functions on all macintoshes (this is a very subtle issue). (this is not "supposed" to happen, but it does. This should be an issue of a different post, it's complicated in its own right).
Hope this helps.
Solution 4:
This is dependent on the USB 3 controller chip located in the drive interface on the external drive.. I am currently going through USB3 hell with this.. Seagate Backup Plus External 3TB does not boot USB3. Seagate Backup Plus Portable 1TB does.. it's all a crapshoot to see which works & which doesn't.. any Mac that has a USB 3 port CAN boot from it, so long as the drive controller/firmware supports it.