Can't shut down/reboot with external drive mounted

Mac 6,1 64GB Ram, D700s, OS X 10.10.3

Can't shut down/reboot system if USB3 SSD drive is connected and mounted. I have to unmount drive before I shut down/reboot.

If I try, screen goes black (does not turn off) mouse is still visible and still moves, but nothing else happens. I have to press the power button to turn off at this point.

I have reset permissions on both the OS and the external drives. I've run Diskwarrior on the external drive...nothing seems to help. I have had the issue on other trashcans in house, but this one is the only one that can't fix the issue.

Any ideas?

This has happened on this system since Yosemite upgrade. Short of a clean install, I'm not sure what else to try. This also happens on multiple users...the external USB3 SSD drive is used as an Adobe cache drive and nothing else is on the drive.