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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?
11-06-2019, 07:48 PM
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