Boot from different drive without using Alt/Option My 2011 27 inch iMac will not boot, it plays the startup sound and then freezes on a white screen. I am fairly confident I could fix it myself if I could get it to boot from a USB drive or external disk but holding Option ⌥ during boot does not bring up the boot selector.

Is there another way to get it to boot from a different drive without using Option ⌥?

For example, if I attach an external drive with OS X installed will it automatically boot from it?

edit: I found this page which sounds exactly like what the problem is. So I am looking for another way to replicate what they are accomplishing, by disconnecting the cable, without having to remove the glass.
If you could boot OS X, you could use the Startup Disk preference pane to select an alternate boot disk.

There are a few other boot-time options you can try: hold C to boot from an optical disk or hold T to boot into Target Disk mode so you can read the hard drive from another computer (over FireWire or Thunderbolt).