Can't boot mid-2012 MBP from external drive

Question

Level 2

276 points

Can't boot mid-2012 MBP from external drive

I have an external USB 3 drive with 2 partitions. One is an OSX 10.8.5 clone and the other is a 10.9.5 clone. My mid-2011 MBP (which is running 10.9) can boot either partition from the external without difficulty.

My newer mid-2012 MBP, however, can't boot either external partition, even though they appear as options in Startup Disk. If I try, it boots instead to my HDD's main partition, which happens to be 10.8.5.

The 2012 has a second, 10.9.5 partition on its HDD. I can boot that fine also. Startup Disk shows both external partitions as options if I plug the external drive in. But if I try to boot to either, the 2012 boots the HDD 10.8.5 partition instead. (The 2012 allows me to browse the content of the partitions.)

I reinstalled 10.8.5 on the 2012 and zapped its PRAM as well -- no help.

Any thoughts on why the 2012 would refuse to boot cloned systems that work fine on the 2011?

Posted on May 23, 2015 4:03 AM

Reply

Answer 1

Best reply

Dave Post Author

Level 2

276 points

May 27, 2015 6:00 AM in response to Dave Post

The solution to my problem turned out to be substituting a USB 2 cable when I need to boot the mid-2012 MBP from the external USB 3 drive.

The main difference between the mid-2011 and mid-2012 is the latter has USB 3 ports. Apparently, the USB 3 chipset in my external drive doesn't allow for booting OSX using USB 3. Switching to a USB 2 cable forces a fallback that works, albeit more slowly.

This switch is needed only when *booting*, though. Cloning OSX from the mid-2012 over USB 3 is no problem for me.

Apparently, when purchasing USB 3 external drives, enclosures, or docking stations (I happen to own a Connectland 3UBT3 dock), one must be careful to inquire about OSX booting compatibility.

Thanks to the posts at Re: Late 2013 13 inch macbook pro will not boot from bootable external hard drive and http://forums.macrumors.com/showthread.php?t=1726319 for cluing me in.

Reply