A flashing folder with a question mark appears when the Mac boots up, indicating that the system cannot find the bootable macOS. 1. First confirm whether the boot disk is connected normally and try to replace the cable or interface; 2. Enter recovery mode to use "Disk Tool" to repair the boot disk or reinstall macOS; 3. Specify the correct boot disk through terminal commands or system preference settings; 4. If it still cannot be resolved, it may be that the disk is damaged or the system crashes, and you need to backup and restore, check the hard disk or send it to repair.

When you see a flashing folder with a question mark when you boot up, you can basically confirm that there is something wrong with the Mac boot disk. This icon means that the system cannot find the bootable macOS system, which may be because the hard disk is not recognized, the system is damaged, or the connection problem is caused.

The following situations are the most common and easiest to solve:

1. Confirm whether the boot disk is connected normally

If you are using an external boot disk, such as an external SSD or a mobile hard disk, first check whether the connection cable is plugged in, try changing the cable, or change the interface. Sometimes it's just bad contact.

If there is a problem with the built-in hard drive (especially old Macs), you can try restarting a few times to see if it will automatically recover. If it doesn't work, continue reading.

• Check the connection cable and interface
• Try different USB-C/Thunderbolt interfaces
• If it is an external disk, make sure it supports macOS and is formatted correctly

2. Enter recovery mode to rebuild and boot

You can fix the boot disk problem through recovery mode. The operation is as follows:

1. Shut down
2. Press and hold the Command R key and then press the power button to turn on
3. Keep pressing until the Apple logo or progress bar appears

After entering recovery mode, open "Disk Utility", select your boot disk (usually Macintosh HD or your external disk), and click "First Aid" to repair it.

If the disk is fine but the system is in trouble, you can select "Reinstall macOS" in recovery mode. Note: Reinstalling the system will not delete your files, but it is recommended to make backups in advance just in case.

3. Set up the correct boot disk

Sometimes the system is still there, but the computer does not know which disk to boot from. At this time, you can use the following method to specify the boot disk:

1. Enter recovery mode (Command R)
2. Open "Terminal", enter bless --mount /Volumes/Macintosh\ HD/System/Library/CoreServices --setBoot
   (Note: "Macintosh HD" should be replaced with your actual disk name)

Or an easier way is:

• On a Mac with normal boot, go to "System Preferences > Boot Disk" and confirm that the correct system disk is selected.
• If you cannot see your system disk, you may need to repair the disk or reinstall the system.

4. What should I do if the disk is damaged or the system crashes?

If the above methods do not work, it may be that the disk is seriously damaged or the macOS has been deleted by mistake. You can consider this at this time:

• Restore the entire system using Time Machine backup
• Connect the hard drive to another Mac and check whether the data can be read
• Replace a new hard drive and install macOS completely

If the hard disk itself is no longer able to be read, it can only be sent to repair. This is not very common, but it is not without it.

Don't be too nervous when encountering this problem. Most of the time, you can do it by restarting, switching the boot disk or repairing the disk. If it really doesn't work, then consider reinstalling the system or hardware issues. Basically that's all, don't panic, just do it step by step.

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