[Solved] MacBook Air internal keyboard and trackpad do not work, but do work in Startup Manager

Andrew Asks: MacBook Air internal keyboard and trackpad do not work, but do work in Startup Manager
2014 MacBook Air, internal keyboard and trackpad stopped working after clean install of Big Sur from bootable USB. But it was running Big Sur fine previously, when I installed it by upgrading from Catalina. Also internal keyboard and trackpad work fine in Startup Manager if I start into it by holding the Option key, which tells me it’s probably not a hardware problem?

If I start to Startup Manager while a bootable USB with Big Sur or Catalina is inserted, and select to boot from then USB (using the internal trackpad!), at some point during the boot I get images of mouse and keyboard on the screen, trying to tell me to turn them on. I cannot proceed after this point.

If I do the same with external keyboard and mouse plugged in, the mac successfully boots from external USB into the screen where I can open Disk Utility, open Safari, or proceed with installing macOS from the USB. But at this point internal keyboard and mouse do not work any more, only the external ones…

Any help will be appreciated.

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