I have a late 2009 27" iMac (10,1) which has 'target display mode'. The Apple website (https://support.apple.com/en-gb/HT204592) suggests I need a Mini DisplayPort to Mini DisplayPort cable and that I need a Mini DisplayPort or Thunderbolt port on my other Mac, but of course the rMB has a USB-C port.... I already have the Apple adapter for USB-C to USB-A, HDMI and USBC, but can't see how that helps. Any ideas if what I want to do is possible and what combination of adapters/cables I need? Thanks.