Can't boot from external with older OS

[vindunn]

Hi,

I’m on an iMac (27in, late 2013) running Mojave 14.6. A couple of years ago, I stupidly updated to Mojave from my older OS and I lost access to important software and peripherals which wouldn’t work with the newer OS. With the help of some folks on the forum, I was able to create a boot disc on an external drive by restoring an older version of my backup on TimeMachine. Again, with help from forum counselors, I was able to boot off the external.

Now, I can’t. The external is visible and can be opened, but it cannot be recognized as a boot drive. Startup Disc onlt recognizes my main drive. I’ve tried all of the recommended fixes (command-R, PRAM, NVRAM reboots) but the boot drive never shows up.

It has occurred to me that maybe the external is somehow gotten corrupted. So then comes another problem: how can I make a boot drive using an older (say High Sierra) OS? I’ve tried to put a High Sierra installer on the external but Apple says it can’t open it.

Any help greatly appreciated. Thanks.

Vin

 

[Cory Cooper]

Hello,

Yes, it could be some form of corruption or the beginning of drive failure.

I assume you still have another external with a current/recent Time Machine backup?

You could try to create a bootable installer of Mojave on another external drive/flash drive, since that is what is currently installed, and then attempt to reinstall over the top of the Mojave installation you have been using.

C

 

[vindunn]

Hi Cory, Thanks for responding. I've had another thought. I've been looking on ebay and I've seen hard drives taken from a Macbook Pro that already have High Sierra installed. So my question is would that serve as a boot drive? Would there be any compatibility problem? I've also seen housing units on the internet that you could put the drive in so that it can be hooked up via USB to a computer.
I would appreciate your thoughts.
Thanks,
Vin