I was recently given a Macbook Air 2012 with a malfunctioning SSD (common problem in this model -- SSD is recognized as 33 kB SandForce disk), so I decided to install and run macOS from a USB stick.

Upgrading to Sierra worked fine with this setup, but when I try to run the High Sierra installer my system drive is not selectable as a target with the explanation that it's missing a "firmware partition".

  1. What is meant by "firmware partition" and what is its purpose?

  2. Is it possible to overcome this obstacle? Can the partition be created manually? Using the original SSD as part of the process is not possible, as mentioned.

Using a USB drive as a system drive is currently an order of magnitude cheaper than buying the obscure type of SSD Apple decided to put in this machine, so that's a requirement. Except if there's a way of fixing the SSD myself of course, but I haven't found one yet.

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