I have a 21.5 inch late 2012 iMac and I wanted to perform a clean clean MacOS Sierra install.  I followed the usual steps in setting up a USB for installation, up to the reformatting/erasing of the hard drive in preparation for the actual OS install.

After following everything I did with previous USB clean installs, I got the error “Installation Fail: The installer payload failed signature check”.  I tried different solutions I found on the web but to no avail.

So, it seems that the clean install from a MacOS Sierra USB no longer works.  So here is what I did instead:

  1. Restore my iMac to factory settings and with the operating system that came with the iMac.
    1. Restart the iMac in recovery mode.  Restart the iMac and hold down the Command and R keys while the iMac is restarting.
    2. Use Disk Utility to erase the hard drive. Click on Disk Utility – Continue. Select the main volume and click Unmount then Erase. Quit Disk Utility (Disk Utility > Quit Disk Utility).
    3. Click Reinstall OS X and continue. Follow the on-screen instructions to reinstall Mac OS X.
  2. Once the original OS has been reinstalled, boot up normally, finish all the setup questions just like when you had a new iMac.  Open the App Store, search for MacOS Sierra, download and install.

This may not be as clean as I have wanted but it’s the closest I can get to having a clean install, not that the USB boot option might no longer be working with MacOS Sierra.