Quando appare il segno di stop all’avvio

Se si installa Trim Enabler con Yosemite si è obbligati a disabilitare le signature delle estensioni di sistema. Se si resetta la P-Ram, il sistema operativo si arrabbia e anziché avviarsi mostra un'icona inquietante. Per recuperare il tutto c'è una procedura ben descritta sul sito di Trim Enabler.

La riporto «as is» perché sono pigro e per averla a disposizione.

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 -u "/Volumes/Your Disk name"

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.