Anyone out there running Shim Di2 -10 speed Ultegra – 6770 series?? Mine came with my new TREK (now 2.5 years old) and I needed to replace the rear derailleur. Bad news is that Shimano no longer stock 10 speed spares, and your only hope is that one comes up on Ebay.
11 speed Di2’s rear derailleurs used to work on 10 speed but apparently Shimano have put a software block on them so that they don’t anymore.
10 speed Dura-ace rear derailleurs don’t work either as the connecting pins are different. I found a couple of 2nd hand ones on ebay – but who knows what condition they’ll be in – and luckily I managed to find a new ‘in-box’ unit from UK which i have just forked out a small fortune for. Still its $1000+ cheaper than upgrading to the current 11 speed Di2.