OK, I made a stupid, stupid error. I bought an iMac for somebody else without a Fusion Drive. Yes, it's crap, but I wasn't using it and they didn't realise the performance should have been better. Frankly, an iMac without either SSD or Fusion Drive is rubbish. MS Word takes +30 seconds to load !! Lightroom takes 45! Even System Preferences takes 10 seconds. So, I wanted to replace the HD with SSD, but of course Apple has closed that door on the 2015 iMacs: at least if I want to keep it under warranty. Next best is to add a Thunderbolt SSD and boot from that. Is this possible? I notice that external Thunderbolt enclosures are not supported on Macs.