SSD w/ brand new clean in stall of Yosemite fails to boot I've just installed a new version of Yosemite on a third party SSD (Crucial). When I boot it up it hangs on a gray screen.

Steps that got me here:


Connect SSD to Macbook Pro (with existing HD) using USB2
Install Yosemite onto SSD while connected to MBP
Complete installation (at this point, I'm up and running with a
working Yosemite running on the external SSD)
Install SSD into internal port
Boot MBP
Immediately hangs on gray screen


I've read about issues with Trim and Yosemite, but to my knowledge I've never enabled or installed Trim on this SSD.

The Option key and Command-R are both non-responsive during startup.
You are most likely suffering from a bad SATA cable. This is a very common issue with any MBP so far, and often presents itself when someone decides to swap out their drive.

You can check this by plugging in both the SSD and an external boot medium and then booting up with option key pressed. If you can see the external medium after some time but not the SSD, your SATA cable is most likely broken. 

This is the thin black cable going to your motherboard from the SATA plug. It's pretty cheap to buy and can be installed really easy. It is screwed down to the chassis so you will have to unscrew it before you can replace it.

Addition to my post: TRIM has nothing to do with this. Neither does Yosemite :) TRIM is a garbage collection technology (or: a ATA command to initiate GC on a specific part). Modern SSD's work fine with or without it. The difference is that TRIM can be used by the operating system to direct some specific garbage collection. This is separate from the SSD controller's own garbage collection scheme. You do not need TRIM, you should not install a "TRIM Enabler" as they work (since Yosemite) by disabling kext signatures and blindly modifying some code and/or a plist for a kernel extension. Some newer enablers use local kext signing as an immediate solution. You will however not have any noticeable performance difference, but you will have a system that's been messed with. The only thing you have to make sure is that you don't have your SSD completely filled 99% of the time as this limits the SSD's ability to maintain itself properly in some cases.