MacPro 2013 Yosemite -- won't start up, monitor never turns on, repeatedly reboots itself

Try disconnecting one monitor.

Is the video card a Mac flashed card? That before did you see images on the monitorduring boot up?

Don't neglect Safe Mode -- it was intended to be available for rescuing you after bad Settings or Bad Additions to Mac OS X.

Hold down the Shift key at Startup. It also does one pass of Disk Utility ( Repair Disk) as it starts up.

The simplest thing your Mac can do is boot with the Alt/Option key held down. The routine to draw an icon for each potentially-bootable Volume is all in ROM. It does not depend on anything loaded off a Drive. It should draw a gray screen even if no bootable devices are attached

Had this exact problem today and was freaking out over it because the machine just wouldn't boot at all and had to be power cycled or would go into repeated self-reboots. In order to see if the problem was with Yosemite, I rebooted with the option key held down to bring up the drive selector, and booted into an external drive which had Mavericks 10.9.4 on it. When everything seemed ok in 10.9.4, I restarted against and booted back into the internal SSD which has Yosemite and the machine has been stable all afternoon. I've seen that for some reason, doing an option key boot can sometimes seem to return the machine into a more stable state - I know its voodoo, but I suspect there's some sort of initialization that happens that doesn't in normal boots.

Im having the exact same problem. Any help/advice would be much appreciated. Maybe 1 out of 10 times, a Reset Password screen will come up but once restarted, goes right back to not working.

I had the same problem and was expecting the worse. I figured I could create a bitstream forensic image of the physical drive since I did not have a recent backup. Mine quite on the install and got caught in the loop. It was a Kernel panic 0 and on the screen it said "OS Not set yet".

I booted the MBP Mid 09 while holding down the option key and there were two options to boot to a device. The one was the install, so I selected that and am letting it install the OS.

The suggestion about unplugging extra monitors was SPOT ON!!

C'mon Apple... this is crazy. Just set display mirroring on installation by default. As Uncle Steve would say "Not that big a deal."

Thanks to

lllaass

Nothing has resolved the issue on mine yet. I was on with tech support for an hour and a half last night. By the end, it will now "start" but it loads to a "Reset Password" screen rather than a login screen. This "Reset Password" screen has three options, however, after trying each one, they all just send me back into the same loop. I just dropped mine off at a local Simply Mac store so we'll see what they can find. (Closest Apple Store is an hour away.)

I had the same problem today. I installed the new system update early this morning, and had it running all day without any problem. (I have a MacBook 2011) But my battery got low and I turned it off later this afternoon. When I turned it back on I went into the same 'Rest Your Password' loop. Nothing I did worked. I went through many of the steps suggested on here, as well as old reliable 'voodoo' options. I was able to get the the Utilities page several times, but to no avail. Finally I realized that the black screen before the reset your password screen is not a just a black screen, but one with a logo. So I did a hard shutdown, holding down the power button. I let it sit for a couple of minutes, then restarted it. As soon as I heard the chime (though faint) I held down the option key, then when I saw the blank off screen go to a pigmented black screen, I released it. At this point I just followed the on screen options, however I could only see them with a flashlight angled on the screen, as the backlight is completely off. I just had to choose my user profile, it installed an update, and let me login as normal. All seems back to stable and normal conditions. Hope this helps!

So have you had luck with trying to view a screen with a flashlight? I keep getting a black screen but with a normal-looking cursor. I'll try another hard reset or two.

I tried booting in safe mode (holding Shift), but that didn't find any issues. I can boot to the 10.9 recovery drive as well without incident, but the Mac just doesn't seem to like Yosemite yet.

Stuart

Michael here,

I just wanted to with in - I updated 5 computers Yesterday ; iMac11,3 - Mac Pro 3,1 - Macbook Pro 8,3, Mac book Air 4,2 - and - wait for it.....

Mac Pro 6,1 - bingo.

The Mac Pro 6,1 is doing nearly the same things "ccano" said - random crashing (multiple monitor setup, older monitors with correct adapters (2 older apple branded monitors, plus Samsung 720p HD TV for video editing).

When I bought the trash can Mac, it came with (I think) 10.9.2 - the OS version that did NOT play well with three monitors - known bug - fixed in next "point version" of the OS - all was well, until yesterday and Yosemite.....

Now Yosemite/trash can Mac is acting the same, glitchy way as the original poster said - three monitors are now an unreliable setup; the only way to get all three working is to reach around back and unplug/plugin the monitors - sometimes that works. I am currently backing up my "Yosemite upgraded" install (via Superduper) in hopes that they will catch this bug, and I can pick up where I left off - I will then "restart" since 10.9.4 Superduper back up is currently running system - some posters have said that simple restart "seemed" to work - I am skeptical, but what the heck - I'll try. If no luck, I'll revert back to 10.9.4 (trouble free for months) until Apple fixes this - Yosemite on the other systems seems glitch-free. Including on the MacPro3,1, running a PCIE SSD startup card and TWO monitors - go figure.

MacPro6,1

64 gigs ram

1 TB SSD internal

(2) ATI / AMD FirePro 700 cards

OSX 10.10 (problem child here - it must be "teething problems"....)

Option Boot makes a temporsy change. To have it "stick" you still need to use:

System preferences > Startup Disk

... and resetting the PRAM erases that change.