IMac will not boot with anything connected via USB

Same thing happened to me couple of times. I powered the HDD on/off couple of times and then restarted the Mac before it reappeared again on Finder.

To further diagnose the problem I would try creating a 2nd user account to log in with, while also disabling any kind of automatic log in if that is enabled for your normal account. This could be used to see if it is something specific to your user account.

Test to see if the machine can restart from the login window or the other user account.

If it does then there is a User account problem occurring, and you can try to manually locate what the problem is by doing manual house cleaning tasks.

If the problem still persists I would try seeing if when you boot your mac into Safe Mode and see if the problem still exists.

To start up into Safe Mode (to Safe Boot), do this:

• Be sure your Mac is shut down.
• Press the power button.

• Immediately after you hear the startup tone, hold the Shift key. The Shift key should be held as soon as possible after the startup tone, but not before the tone.

• Release the Shift key when you see the gray Apple icon and the progress indicator (looks like a spinning gear).

If it goes away then there is some kind of third party driver, etc that may be causing the issue. See more information on what Safe Mode does to find out where to look to troubleshoot next.

If all else fails a very good solution in this case is to backup everything to an external drive, and then do a clean install of OS X. Then you can import just the files you need using Migration Assistant or manually.