How do I make a bootable startup external SSD? SDErrorDomain error 104.

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How do I make a bootable startup external SSD? SDErrorDomain error 104.

I'm trying to make an external SSD that can boot my MacBook, using Big Sur. I formatted the SSD as APFS. I downloaded the app Install macOS Big Sur. The app installed Big Sur on the external SSD. I then change the Startup Disk to the external SSD. When I click "Restart" I get this error:

I tried all this twice and got the same error message. What's wrong?

MacBook Pro 15″, macOS 11.6

Posted on Oct 22, 2021 2:07 PM

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Answer 1

Top-ranking reply

jeffreythefrog

Level 6

12,632 points

Oct 22, 2021 2:20 PM in response to Thomas Kehoe

are you trying to create a bootable installer? if yes, please see this article: How to create a bootable installer for macOS – Apple Support

or are you trying to set up an independent start up drive? if yes, please see this informative, well written user tip: How to Setup and Use an External SSD as your Startup Disk - User Tip

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Answer 2

Oct 22, 2021 2:22 PM in response to Thomas Kehoe

Hello Thomas Kehoe,

Looking over the Apple Support article How to create a bootable installer for macOS - Apple Support, I believe you have the SSD formatted incorrectly.

“What you need to create a bootable installer

A USB flash drive or other secondary volume formatted as Mac OS Extended, with at least 14GB of available storage
A downloaded installer for macOS Big Sur, Catalina, Mojave, High Sierra, or El Capitan“

I hope this helps.

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