Yosemite upgrade stops Macs with aftermarket SSDs from booting
SourceThere’s a new security feature in OS X Yosemite that could render your computer unbootable. If you’ve been using an SSD with Trim support enabled, proceed with caution.
Turning on Trim is a great way to extend the life of SSDs and keep them performing at the peak of their abilities. OS X, however, only supports Trim on Apple-supplied SSDs. If you upgraded the drive at some point, you couldn’t simply switch on Trim and go about your business as usual. Apple’s drivers won’t allow it.