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A safe mode start is done by pressing and holding the left shift key on your keyboard, and then starting the mac while holding the left shift key.
If thats not working, please try the following
Shut down your Mac.Locate the following keys on the keyboard: Command (⌘), Option, P, and R.Turn on your Mac.Press and hold the Command-Option-P-R keys immediately after you hear the startup sound.Hold these keys until the computer restarts and you hear the startup sound for a second time.Release the keys.
And then try a safe mode start again, will it start now?
Dustin S.Senior Technician and Network Switching Systems Operator / Maintainer.