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BillD2751 Author

Level 1

14 points

2019 21.5” iMac won’t boot with new external SSD hard drive.

I just cloned my 2019 21.5” iMac 5400 RPM hard drive to a new OWC Mercury Elite Pro Mini using the free SuperDuper! The process went as expected. I set the start up drive to be the new external drive. Now it won’t boot up from the new drive. It gets about 7/8 of the way and then goes blank. I tried holding the shift key down to go into safe mode but it still won’t boot. How can I get the system up so I can set the internal HD up to be the start up drive and start over? While SuperDuper! was copying files Catalina updated to 10.15.6. I thought I stopped it from updating but apparently not.

Help!

iMac 21.5″, macOS 10.15

Posted on Jul 16, 2020 11:56 AM

Top-ranking reply

BDAqua

Level 10

244,817 points

Posted on Jul 16, 2020 12:47 PM

Hold option key at power up to choose boot device.

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2019 21.5” iMac won’t boot with new external SSD hard drive.

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