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 the 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.
Update
I figured it out. Opened the laptop, disconnected and removed the battery, disconnected trackpad cable. Then connected everything back and reassembled, and keyboard and trackpad started working. Would be interested to know what people think the issue was.
Best Answer
The update provided in the question says to disconnect and reconnect the trackpad. That did fix the issue and allowed me to install Big Sur and use it for 1 day. After that trackpad and keyboard stopped working again. This time dis/re-connecting the trackpad did not do the trick, but dis/re-connecting both the trackpad and the keyboard did! So far the issue has not reoccurred.