How to Diagnose startup problems using safe mode >
1 Start up in safe mode To start up in safe mode, hold down ß immediately after pressing the power key to turn on your Mac. The Apple logo should soon appear, followed by the login window. When you see that, release ß and log in to your user account.
2 Check and try again You can use Disk Utility if you like, or check installed kernel extensions in Terminal (see above). Restart normally. If the problem persists, start in safe mode again and go to your user account in System Preferences > Users & Groups.
3 Rule out Login Items Note what’s listed in Login Items, then use the – (minus) button to remove them all. Restart. Fixed? If so, use the + button to restore items, restarting after each one to see if the issue returns. If so, use safe mode to check for an update.