After experiencing crashes at regular intervals since installing Yosemite-—of late, at least two times a day-—I have stopped using an external SSD as the boot disk in my Mac Mini. In fact, it seems that using any external drive as a boot disk no longer works. Nor can I reliably run my virtual Windows machine from a SSD. It's back to the slow internal drive of the machine.
It works (and I have not had a crash for two days), but I do feel cheated. Something that worked for at least three has been taken away by Apple for no other reason than more profits-—or so it seems to me! As I said in an earlier post, my present Mac Mini will probably be my last Apple computer.