I had upgraded my Mac Mini to Yosemite, a while ago and running without any problems till today. Now it doesn't boot up at all. I removed all peripheral cables (except the monitor), turned off the Mini, waited for 30 seconds or so, turned on (push the switch at the back corner) again, and as soon as I hear the startup sound start pressing the Shift key. First I get a totally blank grey screen. After 15-20 seconds the monitor changes to the black screen and I get the prompt: "no boot device available -- insert boot disc and press any key" What's the remedy? Thanks for the assistance as I'm still a newbee at Mac/OS X.