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I want to install macOS Sonoma from my M3 MacBook Air which runs Sonoma as well on the internal storage. My goal is to install a second OS to an external Samsung T7 SSD and then run the computer from the external SSD.

But I can't get it to work, I have tried for three days now! :(

I have followed [Troubleshooting] macOS Sonoma Can't Boot From An External Hard Drive without any luck. Basically, you download macOS Sonoma from Appstore and choose the install location to an APFS/GUI formatted external SSD.

This is the error I get when trying to boot from macOS Sonoma installed on an external SSD.

The macOS version on the selected disc must be reinstalled.

Screenshot of the error while trying to install macOS Sonoma on an external SSD

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  • Hi! Can you confirm what OS and versions are still bootable from the internal drive on this Mac? Second clarification, you say macOS Sonoma is installed on the external. Which Mac performed that install - the same as you wish to boot or another?
    – bmike ♦
    Commented Jun 16 at 15:26
  • The same computer did the install to the external ssd. I mean after installing it says "need to reboot", but it wont boot from the external ssd. The MacOS is version 14.5 on both internal and the one I try to install. Thank you for trying to help me!
    – raynoralpha
    Commented Jun 16 at 15:35

1 Answer 1

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Since you still have macOS Sonoma 14.5 on the internal disk, you should be able to skip a third party tutorial and follow Apple exact process to install macOS on an external drive.

eschew the left most USB-C port

Pay attention to two things (and the first matters for all updates and install):

  1. Which port you connect the drive for the installation

While you're installing macOS on your storage device, it matters which of these ports you use. After installation is complete, you can connect your storage device to any of them. Mac laptop computer: Use any USB-C port except the leftmost USB-C port when facing the ports on the left side of the Mac.

This port-specificity also holds for DFU type operations which revive or restore a Mac with Apple silicon or the Apple T2 Security Chip.

  1. Which format did you choose when you reach the second step:

I would recommend APFS since you have an SSD. I would also validate the date and time are well correct on the Mac before attempting the next install. You may have a GUID format with APFS, but I probably would erase things entirely with disk utility to ensure it’s correctly formatted and the repair option shows no errors before the next install attempt.

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    Omg. The macbook air only have usb-ports on the left side. I used the left one closest to the power adapter connection. Left one here: apple.com/v/macbook-air/s/images/overview/connectivity/…
    – raynoralpha
    Commented Jun 16 at 15:52
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    Try again with the right - I sure hope this works, I had no clue that existed till just now when I reread the article closely. Samsung T7 should be fine for this and disk utility should complain if it can’t format the drive with the cable and controller you selected.
    – bmike ♦
    Commented Jun 16 at 15:53
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    It is bonkers…. You’d think the installer could just tell you - hey buddy, try the other port, please.
    – bmike ♦
    Commented Jun 16 at 15:57
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    Yes yes yes, it worked! Crazy thing but it worked. Gah after 2 days. Thank you very very much!
    – raynoralpha
    Commented Jun 16 at 16:28
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    Any idea what the technical reason behind this restriction is ?
    – Criggie
    Commented Jun 16 at 23:49

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