Q: iMac 27 5k Retina (10.11.1) won't boot from external
I wanted to boot from an external drive containing Yosemite in order to get Adobe Acrobat to do some work that it cannot do under El Capitán (yes, Adobe is aware there are problems and they "are working on a fix").
But, much to my surprise, I get the "prohibit" sign regardless of what version of Yosemite (10.10.5, 10.10.3, 10.10.3 recovery or 10.10.1 recovery) I offer and regardless of connection (USB 3 or FireWire through Thunderbolt adapter).
All disks are recognized as bootable if I hold the "Option" key after the chime. The 10.10.5 and 10.10.3 are also recognized as bootable by System Preferences' Startup Disk. They can also be accessed as drives if mounted. I have checked for any disk errors via Disk Utility and there are none.
I am stumped because there are no intermediate hubs involved as reported in another thread I can't find right now, but I could swear that this machine did start on one of them when I first got it less than a month ago (I think, but I could be wrong!).
Now, I have not tried other more exotic methods (such as those discussed here), but booting from these drives worked with an older iMac 27 (early 2011) so why doesn't it work now? Is this a feature or a bug of El Capitán?
Posted on Nov 29, 2015 8:48 PM