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Can't complete Catalina installation process

Hello,

I have a late 2013 21.5" iMac, and until this morning I had been running Mojave on the internal HD, and Catalina on an external USB SSD.

I received a notice of a security update for both Mojave and Catalina, which I installed. When my iMac restarted, it would only reboot into Mojave, even though the Catalina SSD was recognized in the Startup Utility.

After several frustrating attempts to get Catalina to reboot, I erased the external SSD to do a clean installation of Catalina. The installation begins, but when the iMac restarts, it restarts using the internal HD (Mojave), and will not complete the installation of Catalina.

This had been working flawlessly for several weeks until I applied the update. Any ideas as to how I may complete the clean installation of Catalina to my external SSD?

Thanks in advance,

Bill

iMac 21.5″, macOS 10.14

Posted on Dec 15, 2020 8:11 AM

Best reply

William Tomcanin Author

Level 2

253 points

Posted on Dec 16, 2020 11:36 PM

Problem solved after removing all connected peripherals, multiple NVRAM resets and downloading a fresh Catalina Installer.

After I was able to successfully install and reboot, I summarily ignored the notice of a security update and am running normally again.

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Can't complete Catalina installation process

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