I have a few months old 8 core MacPro D700s 32GB RAM that came with El Capitan pre installed. Works fine. I have two monitors hooked up: a HP Z27q 5K monitor (via two TB cables) and a Dell DEP2214H widescreen. There are a couple large capacity thunderbolt drives. I have attempted to install Sierra twice and failed. I though I'd check in with the MacPro experts here to see what I'm missing. I have been using Macs for decades; its not my first OS update/install rodeo The first time I elected the upgrade path. It got as far as the login screen background (without the login prompt) and stalled (for hours). There was an unable to unlock keychain and a couple color space errors in the install log. I recovered from a TM backup and things were fine. Since there have been so many success stories, I thought perhaps it was a fluke or operator error. The second time I first created a partition for Sierra (as you would for dual booting), downloaded a fresh install image and installer, and pointed the installer there. After the first restart, the system displayed blank black screens but the voice direction was heard. It was kinda hard to navigate with black screens, but things like arrow keys seemed to be responded to (via voice) but I couldn't figure out how to proceed or cancel/back out. I did not get any voice prompts the first attempt. I rebooted via the power/shutdown button but found the machine would not boot, wouldn't get past the chime and external discs spinning up, nothing displayed, no flashing ?, nothing. I waited 5 minutes. I tried holding the option key while restarting.. but nothing. I was able to reboot into the local recovery partition, set the startup drive to the El Capitan drive, restarted and thinks are fine. From the finder I can see OS stuff on the Sierra partition (no user folder) but system preferences won't allow me to set Sierra as the startup disc. Not surprising but just a bit of info. Any clue whats going on here? Will the third time be the charm? I've heard rumors that Sierra has problems with some monitors, mostly wide screens. But that Dell has been around for ages. If I get some ambition, I may try disconnecting the HP and see what happens, but it took so long to get the OS to work right with them. Or maybe I'll call Apple support first. Its been awhile since I've had the pleasure of talking to them.