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cannot startup from External Hard Drive

2020 iMac 27 inch with Seagate external hard drive plugged in to usb. Prior to Big Sur I could boot from the external. Won't let me do that anymore. Why and how can I fix it.


iMac Line (2012 and Later)

Posted on Dec 23, 2020 1:41 PM

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Posted on Dec 23, 2020 2:09 PM

You need to enable booting from an external drive.

Start your mac from Recovery by holding down command-R, then select Startup Security Utility and check the relevant box. Restart holding down the Option key as usual and now you should be able to select the drive and boot from it.

7 replies

Dec 23, 2020 1:51 PM in response to SandG__

I've heard that macOS Big Sur does, in fact, no longer allow booting from an external drive; for security reasons.


I believe there is a fix but only by reducing security.


But I'm not sure what that solution is; can anyone advice? I think it's a case of issuing a Unix command from Terminal in single-user mode.


UPDATE: I think this is it: https://support.apple.com/en-us/HT208198

cannot startup from External Hard Drive

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