If you're using running a Mac that uses a third-party SSD, then upgrading to OS X Yosemite "will leave you in a world of hurt as Apple drops support for third-party SSDs," reports ZDNet (http://tinyurl.com/nrxxltm).

According to Cindori Software, makers of the popular utility Trim Enabler, the issue involves a newly introduced security feature called kext signing. A kext is an OS X kernel extension, or a driver. Enabling TRIM is one of the best ways to maximize the life of a solid-state drive, but OS X doesn't support it out-of-the-box except for Apple's own drives, but until now it has been possible to enable it by altering drivers. 

"Kext signing basically works by checking if all the drivers in the system are unaltered by a third party, or approved by Apple," Cindori Software writes (http://tinyurl.com/ojmjmc4) on its blog. "If they have been modified, Yosemite will no longer load the driver. This is a means of enforcing security, but also a way for Apple to control what hardware that third-party developers can release OS X support for."