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I have an iMac 2013 with HDD and I recently upgraded it with extarnal SSD connected via Thunderbolt. It works pretty well but sometimes when I turn my iMac on, it shows a black screen with text like "Can't boot, press key or wait a few seconds for reboot". So then it boots as expected. Any ideas why this happens and how to fix it?

A few notes. Once I tried to boot with "Option" key pressed to choose bootable disk and there was only HDD in list so SSD didn't appear at all. My SSD is always connected and its activity indicator lights up immediately after I press start button.

Looks like I need to delay startup for a few seconds to let the SSD be recognized but I don't know how to to that.

Would be glad to get any suggestions, thanks!

  • You can have a startup delay if you install a free copy of rEFInd. – David Anderson Dec 6 '18 at 10:57

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