Mac mini with OSX on external hard drive crashing upon boot
I have a Mac Mini with OSX Lion, which was installed on a passport external hard drive, and plugged into the Mini.
Was working 3 weeks ago, but now when I power on, get chime, mac logo, and long progression bar on display. As soon as it reaches 100%, powers off instantly.
Tried holding down shift key upon restarting, but nothing changes.
Help, I really would like my Mac back...
Mac mini (Mid 2011), iOS 7.0.6