Macs may freeze and annoy users with a blue screen when starting up, and reboot may be just a temporary solution. In this case, it is difficult to determine cause-and-effect relationships, because there are several reasons for that.
However, the first thing to do is to find out whether this issue is hardware or software related.
A Safe Mode Boot
A Safe Mode will help with identifying the source of the problem by removing the caches and checking the system.
- Start a Mac by pressing a power button, and holding the Shift key until a sign-in window shows up. If your startup disk is encrypted with FileVault, you will be asked to enter your username and password.
- If everything goes smoothly when starting up in Safe Mode, you should check up startup items and fonts, which may cause problems.
Find the following folders via Spotlight and place them on your Desktop:
It will not harm the system. Reboot your computer in a normal mode. If you do not experience any problems with your Mac freezing at the blue screen, it is a startup item or font causing the issue.
If you still see a blue screen, it is time to test your hardware.
Check Your Peripherals
Any USB or Thunderbolt device may be the reason for the problem. Unplug all external hardware and try to boot your Mac again. When the login screen or desktop is displayed, you can attach peripherals one by one to see if some of them will still cause problems.
Then you can shut your Mac down and try adding peripherals before pressing the power button. When the culprit is detected, replace it with another one.
Repair the Startup Drive
If the external hardware is fine, try repairing the startup drive with Disk Utility. Of course, it is impossible to access it without booting your Mac, so you have to do it from another disk with an operating system on it. Then open Disk Utility via Spotlight or Finder (you can look for it in the Applications folder).
With this tool, you may isolate the problem if it is caused by your hard drive. If you find an error, contact Apple support.
Change Startup Drive Permissions
If the previous options do not help, try this one.
- Press the power button while holding down the Command + S keys combination until several lines of scrolling text show up.
- When the text stops scrolling, enter the following lines, pressing Return after each of them:
mount -uw /
chown root /
chmod 1775 /
Your Mac will start up from the startup drive. If the problem persists, your system must be reinstalled.
Reinstalling the Mac OS
Reinstall your operating system without erasing the drive. For this, you have to boot to the Recovery hard drive:
- Press the power button, and after the chime press and hold down the Command + R keys combination. Wait until the Apple logo shows up, and then release them.
- Choose macOS Utilities and Reinstall macOS without erasing your startup disk.
- Then follow the instructions. Your computer will automatically reboot when the process is complete.