New iMac won't boot from external USB drive

[wangl2]

Hi All,
I always carry a bootable USB drive which has OS X 10.9.5 installed for my troubleshooting and quick reimaging job if I need to. Recently I received some iMacs which came with OS X 10.11. And I found when I connect my USB drive and tried to boot from it, it won't.
Is there any new security feature in the iMac that's causing it?
Thanks.

[howie_isaacks · Accepted solution]

As far back as I can remember, Macs won't boot (at least properly) from an OS that is less than the one they were preloaded with when they were new. You should update your boot drive to at least 10.11 to ensure that will boot most Macs that are recent generations. Also, keep in mind that an older OS booting a newer Mac may not have all of the device drivers needed to boot the newer Mac that was shipped since that OS first released.

[wangl2]

I will try to update my bootable USB drive. But I never had problem in the past. I used to have a 10.6.8 OS X bootable on a USB drive and she always worked with all my iMacs, including the ones that shipped with 10.10. Only started to have problem with 10.11.

[wangl2]

Thanks Howie. Updating my Bootable USB drive to 10.11 worked.

[EddyLara]

Hi How do you update your Bootable USB Drive?

[howie_isaacks]

I just boot up one of my Macs using the USB drive and run the software updates.

This is what I use as a boot drive.

https://eshop.macsales.com/item/OWC/ENVMKU3S240/

There's a 480GB model, but the 240 is sufficient. I have it setup with two partitions. One is a normal boot volume that allows me to run a Mac normally, and use various tools that I have installed, and the other is setup as a macOS installer. These drives are very fast.

[wangl2]

@eddylara72 just boot into your USB OS, and run software updates.