Mac Pro (10.11.5) can't log in, doesn't display screens I have an old Mac Pro tower (2009; firmware 5,1). I just updated to the latest El Capitan OS X 10.11.5 from 10.11.4. Since the upgrade, the computer has not been displaying screens (I have an Nvidia Quadro K1200 graphics card). However, if I use screen sharing from a different computer I can see the log in screen. However, if I try to log in, I can not. If I type in an incorrect password, the little typing window immediately bounces left/right. If I type the correct password, the typing window disappears, to be replaced by this spinning wheel (shown here in very low quality):



This spinning wheel usually means a successful login. It spins for about 5 seconds and then gets replaced by the login window again.

Any thoughts or suggestions?
I figured out a way to solve this problem without too much trouble. However, it only works with a second mac computer.


Put the Mac Pro into target disk mode.
Use a cable to hook it up to another mac computer (I used a firewire 800 to thunderbolt cable)
Restart other mac computer and hold down option to see boot options.
Boot other mac computer from Mac Pro's startup disk.
Follow startup/login process as usual.
Restart both mac computers.


The Mac Pro now runs as usual.