How to fix when Mac fails to reboot, instead hangs at a black screen, while unplugging USB devices continues boot? 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?
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.