I am using two Dell Ultra Sharp HD Monitors UP2414Q with my Mac Pro (late 2013) single processor quad core running el cap 10.11.1 with AMD vFirePro D300. The have been working seamlessly with Yosemite since plug and play install in April. Last week the Pro began getting hung up upon boot up after 1/3 of the progress bar filled under the apple logo. The next thing seen was the generic arrow in the Right upper corner of the L monitor. The R monitor never initialized. They are connected via thunderbolt (mini Display Port) to display port. The only thing that will rescue is a hard reset with power button. I can also make it work by using HDMI out on Pro to HDMI in on Display. No crash then. However, I can only use one Display. I have tried Thunderbolt cable to mini display port input on Monitor, I have tried mini display to HDMI on monitor. Any of these hangs the boot up. Can anyone suggest a diagnostic step or possible fix? Thanks.
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