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I almost never reboot this Mac. Lately I noticed after a system freeze that the Mac would not boot, instead the Mac boot process hangs at a black screen.

Regression:

  • Unplugging the USB devices immediately continues the boot process to the gray screen with the Apple logo.
  • Because 2 USB hubs are attached I tried to isolate, by moving all USB devices to one USB hub and then to the other, no improvement.
  • Tried to isolate the individual USB devices, boot still fails.

How to fix the boot process of my Mac?

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After a few hours of fiddling with USB and USB devices I found the solution.

In "Apple menu" > "System Preferences" > "Startup Disk" there was no longer a disk selected that I would like to use to start up my computer from.

After selecting the corresponding Mac OS X system, I could reliably reboot again, without pausing/hanging at the black screen during boot.

Startup Disk Select the system you want to use to start up your computer 10.9.5

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