disk erase failure, couldn't unmount disk?
How do I erase my HD clean if I'm getting this message on my 2012 mac Disk Erase Failure, Couldn't unmount disk
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Are you trying to erase your boot drive? you can't do that - the app you're using to format the drive (presumably Disk Utility), as well as the active OS is on that drive, so you can't, erase it.
You have to boot from some other drive to do this.
If your system is recent enough (e.g < 5 years or so), then you can boot into Recovery mode (Hold Command-R on power-on), from where you can boot a minimal system that should allow you to do this.