Posting here as an FYI, as well as a query for help if someone has a solution. Sierra broke my external VGA monitor connection; it's mDP->VGA adapter. I'm booting Sierra off an external HDD connected vis USB to my late 2013 15" rMBP, & I have also a 27" ACD connected. When I boot from Yosemite, from my internal SSD on my rMBP, all 3 displays work fine; the rMBP recognizes both the ACD as well as my old Viewsonic display connected via a mDP to VGA adapter. Trued SMC & PRAM resets, disconecting & reconnecting, rebooting, detecting displays by pressing the Option key, etc etc.....to no avail. The fact that all works OK (quasi-OK: my external Viewsonic is only recognized in lower rez than its actual 1680x1050) on Yosemite means it's a Sierra problem, not a rMBP problem, right? Thanks to all who participate in these forums, I learnt so much from you guys, appreciated!