About Trim in Yosemite
You will want to read this article by Cindori AB. I am posting the code from his article just in case this page goes missing for some reason.
Recovering from stop sign on boot screen
For those who are stuck on the grey boot screen, here’s how you get back into OS X:
Step 1: Boot recovery mode by holding Cmd+R during boot
Step 2: Is your volume encrypted (FileVault)? If so, first open Disk Utility from the menu bar and unlock/mount your drive
Step 3: Open the Terminal from the menu bar
Step 4: Run this command:
nvram boot-args
Does it say “kext-dev-mode=1″? if so, you can skip to Step 6. If it says “error getting variable”, continue with these steps:
Step 5: Run this command:
nvram boot-args=kext-dev-mode=1
Step 6: Reboot back in to Recovery Mode again
Step 7: Run these commands, replacing Your Disk Name with the name of your Mac disk (partition). You can type ls /Volumes to get a list of volumes. Note the quotes around the disk path and that there should be no / before System.
cd "/Volumes/Your Disk Name"
touch System/Library/Extensions
kextcache -prelinked-kernel System/Library/Caches/com.apple.kext.caches/Startup/ kernelcache -K System/Library/Kernels/kernel System/Library/Extensions
Step 8: Wait until it finishes (can take as long as 5-10 minutes, don’t abort it) and reboot.
You should now be back in OS X with Trim enabled.
Reversing any changes by Trim Enabler
If for some reason the above steps does not work for you, you can undo all changes made by Trim Enabler by following these steps:
Step 1: Boot recovery mode by holding Cmd+R during boot
Step 2: Is your volume encrypted (FileVault)? If so, first open Disk Utility from the menu bar and unlock/mount your drive
Step 3: Open the Terminal from the menu bar
Step 4: Run these commands, replacing Your Disk Name with the name of your Mac disk (partition). Note the quotes around the disk path and that there should be no / before System, except for the first part of the cp command.
nvram -d boot-args
cd "/Volumes/Your Disk name"
rm -rf System/Library/Extensions/IOAHCIFamily.kext
cp -r /System/Library/Extensions/IOAHCIFamily.kext System/Library/Extensions/IOAHCIFamily.kext
chown -R root:wheel System/Library/Extensions
chmod -R 755 System/Library/Extensions
touch System/Library/Extensions
kextcache -prelinked-kernel System/Library/Caches/com.apple.kext.caches/Startup/kernelcache -K System/Library/Kernels/kernel System/Library/Extensions