Weird: External HDD will not boot...

[komatsu]

I have an external HDD here running macOS which has a few diagnostic tools on it. Generally, it never fails me. I press ALT and select disk and the OS loads and I can run my tests.

However, there is a mid-2012 iMac here and each time, I select ALT it sees the disk. But the moment I select it I get the prohibitory symbol.

If I try to run Apple Hardware Test by pressing "D" at startup all I get is "choose network".

Very frustrating. Anybody know what the problem could be?

 

[HDFan]

You boot the mac from the disk? If so what OS is on the disk, and what is on the iMac?

 

[Fishrrman]

The "prohibitory" symbol (red circle with a line through it) generally means that the -version- of the OS you're trying to boot from (on the external drive) is incompatible in some way with the Mac.

Questions I'd be asking:
- What VERSION is the copy of the OS on the drive?
- What version of the OS is currently installed on the 2012 iMac?
- Assuming YOU HAVE NOT MADE ANY CHANGES to the OS on the external drive, has it booted the iMac in question before?

 

[komatsu]

The system was running High Sierra. The external drive has El Cap on it.

But the problem is solved.

Here is the solution:

I created a new boot up disk with El Cap on it.

Installation was going great. Then I got the "installation almost complete" (or something to the effect) message. "1 second remaining..." Then I got. The "os not eligible" message. So close yet to so far. I was back to square one. Very disappointing.

Back to Uncle Google. Loads of people giving same advice about making sure the date is correct on your system. It was! However, I came across one poster who said that "out-of-date certificates" were blocking macOS installers. This got me thinking. I decided to change the date on the computer back to 2016 using Terminal. Installation went perfectly and El Capitan is now installed on system.

I have little doubt that Apple set this little quirk up deliberately to thwart users from upgrading or refreshing their systems. They want you to buy a new Mac. TBH, Apple are making Microsoft looking like Mother Theresa as each year passes...

Thank you HDFan and Fishrrman.