How to boot up in safe mode?

To boot into Safe Mode you restart your mac while pressing and holding the shift key.

But that won't allow to restore your mac from your backup drive.

If your backup was made by Time Machine restart your mac

while pressing and holding the Command and R keys.

Your mac will boot into Recovery Mode from there you

can select Restore from Time Machine.

You do not need to boot into safe mode to restore from a backup drive. Are you sure this is what you need to do?

Boot Into Safe Mode

1. If your Mac isn’t already shut down, then shut it down and wait at least 30 seconds before rebooting.
2. Immediately, at or before the chime, press and hold down the SHIFT key. 
3. Release the key after the Apple logo and progress bar appear.
4. Safe mode startup is much slower than normal startup, so be patient.
5. When the Login Screen appears enter your admin password. If you use automatic login, then this means you are in safe mode.

If your backup drive is bootable, then you should do this, instead:

Boot Using OPTION key

1. Restart the computer.
2. Immediately, at or before the chime press and hold down the "OPTION" key.
3. Release the key when the Boot Manager screen appears.
4. Select the disk icon from which you want to boot.
5. Click on the arrow button under the drive icon.