Unable to use bootable USB flash drive as startup disk

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Level 1

14 points

Unable to use bootable USB flash drive as startup disk

I have a MacBook Air (early 2020) with macOS Catalina (10.15.7) installed. I am trying to install High Sierra to a volume on my Mac so that I can run some 32-bit Mac games. And no, running High Sierra as a VM will not allow me to run the games in question. I set up a SanDisk USB flash drive as a bootable drive. My USB drive shows up as a bootable drive in Startup Disk in System Preferences. However, when I try to restart using the USB drive, or when I try rebooting my Mac while holding the option key and selecting my USB drive, I get the following message: You can't change the startup disk to the selected disk. The startup disk could not be verified. Select another startup disk.

I have tried several times with one USB drive, and then tried with another. I get the same result every time.

Can I do this? If so, what am I missing?

Thanks for your help.

MacBook Air

Posted on Oct 28, 2020 9:53 AM

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Answer 1

Top-ranking reply

Eau Rouge

Level 8

42,590 points

Oct 28, 2020 5:15 PM in response to downeaster59

The simple answer is that if your mac came preinstalled with Catalina it cannot install an older OS

even if you want to install it to an external drive.

No mac can install an OS older than the one it came preinstalled with, which also includes

any external disks.

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Answer 2

Barney-15E

Level 10

116,218 points

Oct 28, 2020 3:27 PM in response to downeaster59

You can’t install High Sierra on an APFS volume unless you start with High Sierra and add Mojave or Catalina to a Volume. The High Sierra installer won’t recognize the Volume. It would likely work on a partition if the installer would boot.

I don’t know why you are getting that error. I’ve booted a High Sierra USB installer without issue.

if you have a back up of your Catalina install, you could erase the disk and install High Sierra, create a volume on that APFS Drive, then install Catalina and migrate from back up.

You would probably need to use Internet Recovery to install the OS that shipped on your Mac, upgrade to High Sierra and go from there.

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Answer 3

downeaster59 Author

Level 1

14 points

Oct 28, 2020 3:35 PM in response to Barney-15E

I figured out why that error was happening, but I still couldn't install High Sierra on the volume. When I began the installation, after the Apple logo appeared, a logo of a circle with a line through it appeared. So, it seems that I can't in fact install High Sierra in this way, as you said!

Could I install Mojave by the method I've been using, or is the only way to install Mojave or High Sierra the one you outlined in your last paragraph?

My Mac is an early 2020 Air, so it shipped with Catalina.

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