Booting from USB 3.0

I have question about booting from a USB 3.0 device. I am using a Mid 2012 retina MBP 15inch. I have an external hard drive with Mac OS X installed on it. I can boot to that OS just fine when the hard drive is attached to my USB 2.0 external enclosure, but when I attach the drive to my USB 3.0 enclosure the drive isn't recognized when I boot up the mac and hold the alt key (all internal dirve partitions are recognized OS X 10.8.4, Bootcamp, and Recovery appear). The drive is recognized and I can explore the contents in Finder when attached to the system after booting to the OS on the internal flash storage.

Someone else in these forums had the same issue with a USB 3.0 device not being recognized at boot. I want to know if this is a Mac issue or my USB 3.0 external enclosure problem? Has anyone been able to boot to a USB 3.0 device or is this not capable with the USB 3.0 capable Mac devices? If anyone could provide any info on this problem it would be greatly appreciated. Thanks.

MacBook Pro (Retina, Mid 2012), OS X Mountain Lion (10.8.4)

Posted on Jun 25, 2013 5:25 AM

19 replies

Al Q

Level 1

124 points

Mar 14, 2014 2:36 PM in response to cbs20

I have just gone through very extensive tests with USB3 external drives and my mid 2012 Macbook Pro (non Retina).

The short answer is that if your Mac has USB3 hardware, you can read and write via USB3 when booted into Mac OS, and take advantage of its greater speed. Some people have reported that you may be stuck at USB2 speed if the first device connected to the port is USB2. But you can always fix that by just rebooting.

HOWEVER the boot ROM of many of those Macs does NOT support USB3. To boot from a clone you have made on a USB3 drive, you need to connect your drive via a USB2 hub or USB2 cable. It will operate at USB2 speed but it will work.

I have submitted a bug report to Apple, but there is no guarantee that they will choose to develop updated boot ROMs that support USB3 for older Macs with USB3 ports.

Mar 14, 2014 2:48 PM in response to Al Q

Yes and no.

Your point about booting via USB 2.0 hub is spot on. But

(a) This is not just about older macs, because my 2012 retina MBP used to boot from USB 3.0.

(b) Some enclosures work and others don't any more, and

cbs20 Author

Level 4

2,796 points

Mar 14, 2014 3:55 PM in response to tryitonabreadboard

Right. I just picked up a new USB 3.0 enclosure made by Anker and I could boot to a drive connected to that enclosure, but as I stated earlier, another USB 3.0 enclosure I can't boot to. It just seems like Apple has half assed this feature and don't have fully compatibility with all brands. I submitted a report to Apple as well.

Al Q

Level 1

124 points

Mar 15, 2014 1:56 PM in response to cbs20

Half baked boot ROM support for USB3 does seem to be the problem. For my vintage there is no support at all. For some newer models there is partial support. At least one person has reported being able to boot in USB3 mode on a mid 2013 Macbook Air if the external drive is unencrypted, but not if it has been formatted as encrypted using Disk Utility.

When it comes to generic external enclosures though, don't blame Apple for all the problems.I think a lot of the cases have buggy firmware. I tried several 2.5" enclosures before finding one that worked reasonably well with Mac OS running. But neither it nor a name brand (Seagate) USB3 portable showed up as a boot option after restart with option key.

This thread has been closed by the system or the community team. You may vote for any posts you find helpful, or search the Community for additional answers.

Booting from USB 3.0