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I have a boot hard drive from an old Mac and a new Mac. I want to boot the new Mac from the old Mac's boot disk (preferably without having to take the new Mac appart) So I attach it via USB, configure it to be the boot drive in System Preference and reboot. Almost immediately after the chime, the new Mac displays the grey prohibition sign 🚫.

The new mac is running 10.14. The old Mac that I'm trying to boot into is on 10.13.

Any ideas as to the cause of this? How about ways to troubleshoot?

According to the internet, booting from external drive is possible these days. And the prohibition sign appears when a bootable drive is found. So I don't know what's the problem.

  • Quite possibly 10.14 is the lowest your Mac can run. Macs cannot run older operating systems than what they were released with. If you have an upgrade disk, use it to upgrade your external drive. Also, remember that attaching via USB can be a lot slower than attaching internally via SATA – Matthew Barclay Apr 28 at 1:27
  • Good guess @MatthewBarclay, but the "new" Mac is not that new, actually. It used to have 10.13 on it. – user2026920 Apr 28 at 1:50
  • What happens if you hold Opt while booting? Does it detect the drive? Also what are the specific models of both the old and new Macs? – Allan Apr 28 at 10:59
  • @Allan, es, when I hold Option, the computer displays the the option to select between the two bootable drives: it's own internals and the old Mac's via usB. The old Mac was 2008 Mini (I don't have the actual model number) The new is 2013 A1502 – user2026920 Apr 28 at 18:11
  • Does it boot from the USB in this case? – Allan Apr 28 at 18:39

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