Installing macOS on external drives: not all ports work!

TidBITS Talk
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I just stumbled on a port restriction for installing macOS on external drives: Install macOS on an external storage device and use it as a startup disk - Apple Support

If you’re using a Mac with Apple silicon, plug your storage device into any compatible port except the DFU port. Learn how to identify the DFU port. After installation is complete, you can connect your storage device to any compatible port, including the DFU port.

I had no idea that you need to avoid the DFU port!

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I love how there is no end-user facing consistency to which port is the DFU port across models.

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New to me as well. Very surprised there is no consistency or even just some visual indication.

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I can understand why that port is special for DFU. In DFU mode, the system is not running macOS but some stripped-down OS (permanently burned into ROM) designed only for supporting DFU operations (e.g. install T2 firmware, install pre-boot firmware, install pairing keys for storage modules). As such, it is probably wired differently from the other ports.

I don’t know why that port can’t be used for a macOS installer, but if macOS requires a special device driver for that port (assuming it is wired differently), then the not-macOS system that runs during installation may not have the driver necessary to use that port.

But I agree that there should be some marking to indicate that this port is different from the others, even if you have to visit Apple’s support site in order to understand how it is different.

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I also just stumbled on this new info. I have followed the external booting saga on Silicon macs in forums, anrticles and by actual use and testing since Silicon macs were introduced in 2020. I feel this must be something Apple has recently slipped in. With all the thousands of threads and articles about external booting I have never seen any mention of not using the DFU port.

The other factor suggesting it is a recent thing is that I have done many many installs on externals on my MBAs over the last four years with no regard for the DFU port. Yes I had a few failures but feel I must have been extremely lucky to always have randomly selected the non DFU port.