I'd try a new SATA cable and another SSD.
At Christmas I updated my failing Mac mini original hard drive to a SSD, and upgraded to Yosemite at the same time. I used the Trim enabler app to enable TRIM and disable kept signing, this worked well. Sporadically since then on boot, the no entry sign will show for a few seconds, but will then disappear and the boot will continue as usual.
however, a few day ago, in an attempt to fix a much lesser issue of a mission control desktop being created every time I boot, I started the Mac in Safe Mode. This was fine, but when i then tried to restart in normal mode, the stop sign came, and stayed.
I Tried to follow Cindoris instructions on how to recover from this issue via Terminal commands in Recovery mode, but it would always get to the last command before it came up with an error message along the lines of something not being able to be found.
I Keep a Time Machine backup on constantly and following the issue, I went into recovery mode and recovered to an earlier, safe time.
On completion, when the Mac auto booted, the no go error sign came, but thankfully went.
I Then used the Trim enable app to turn on trim, which it said was off, and on the restart it does to disable kept signing, the error sign occurred!
Any help would be much appreciated!
Mac mini, OS X Yosemite (10.10)