Installing HS... Error Message "Failed to Write "

I have tried to install High Sierra on my 27" iMac w/16GB memory, but I keep getting the message "Failed to write file <Private>" and the process stops. I've tried going into recovery mode, but I get the same thing. The only thing I haven't tried is reformatting my hard drive, installing High Sierra and then restoring my files from TimeMachine.

Any help before I take the nuclear option?

Thanks