Less than two months after my previous catastrophic without warning mac fail (see here: http://forums.macrumors.com/threads/mac-wont-boot-no-warning-given-fsck-good-so-whats-up.1953921/ ) and I'm back watching grey screens fail to load. This time I can't get to any of the special modes - command S gives me some code then freezes, T gives me the target disk symbols but the drive doesn't appear on the exterior computer. I'm thinking that maybe the apple techs who replaced the logic board (see previous post) may have put something back together incorrectly. Maybe a cable popped out? Maybe a ram stick fell out? I was running nothing special when it happened. In fact I was using the computer to generate a single tone in ableton. Kind of funny when you think about it. It's hard to imagine a less intensive task. I suddenly got the beachball across the whole computer. I rebooted and got a grey screen with the "prohibited" ie zero with a cross through it. It's like the universe doesn't want me to make music or something. Last time it failed was when I was sitting down to a period of working on music also. wtf!!!! --- Post Merged, Mar 24, 2016 --- Maybe the tone was at the correct frequency to vibrate something internally and the hard drive plug fell off the logic board? Does that sound crazy? Please send some ideas.