We got several new iMacs this past Friday (10/31) which all came with not what was ordered, instead with 10.11. Have done everything including destroying the Fusion drive and recreating it so that no possibility of 10.11 exists on either the SSD and HD. Still the machine will not except Yosemite or any other OS older than 10.11. Even after CCCing 10.10 to the rebuilt Fusion, still will not accept the OS. Called Apple and they said once its been factory installed you cannot go back. Must be a way around this..................... Any input would be appreciated.