What options do you have?
The only way to allow TRIM to work with third-party drives under Yosemite is to disable kext signing. Unfortunately, you can't do this on an extension-by-extension basis. Instead, you have to turn it off globally. And when you do, this opens the security hole that Apple has attempted to close. Because I think it's a terrible idea to disable this security measure I won't provide you with the steps necessary to make it happen. (There's a big wide Internet out there that will guide you if you want to pursue this course.)
Another option is to get an SSD that uses the SandForce processor, which some claim makes TRIM unnecessary. OWC's Mercury line of SSDs use this processor.
You could also take your Mac to Apple and ask them to install an SSD that will work with their driver.
And finally, you can send Apple feedback and suggest that in a future update they make Yosemite compatible with TRIM and third-party SSDs.
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