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Known Mac Error Messages: Their Backgrounds & How to Solve Them

You start your Mac, but there is only a folder with a question mark or a prohibition sign on the screen? We present these and other Mac error messages to you in our guide. We'll also tell you how you can fix the problems that trigger these messages.

The prohibition sign symbol: startup volume cannot be used

You turn on your Mac, but a prohibition sign appears on the screen? This means that the computer cannot use your startup disk. This is a partition on the hard drive that contains the macOS operating system. In this case, restart your computer, press the ALT key on your keyboard and try to select a different startup disk. If you can't fix the problem this way: Reinstall macOS. Use the key combination when restartingOption + ⌘ + R and let go as soon as you see the Apple logo. Then the utility opens, in which you select "reinstall macOS". However, if there is a mechanical failure, it is probably necessary to repair or replace your Mac's hard drive or SSD. The best way to do this is to contact Apple's customer service.

The folder with question marks: Startup Disk is not available

If you see a folder with a question mark on your computer display, the startup disk is currently unavailable. The best thing to do is restart the Mac, hold down the Alt key on your keyboard and select your startup disk. If that doesn't help, you should reinstall macOS. Or contact Apple Customer Service with your problem, because they may be able to help you.

Your Mac's startup disk is almost full

The more data you save on your Mac, the less space is left for swap files, which can affect the performance of your computer. If the message "The startup disk is almost full" also appears, you should do the following:

  • Check your desktop or the download folder to see if there are files there that you no longer need - if so: delete them.
  • Then open your trash and empty it completely.
  • Click the apple icon in the top left and select "About This Mac". Under “Hard disks” you can find out what data is in the memory. Click on "Manage" for more details. Maybe you will find more photos or documents that you no longer need.

About error codes on the Mac

Sometimes it happens that you receive an error message with a code on your Mac. Most of them, especially newcomers, can hardly do anything - especially since the dialog window does not always provide clear explanations as to which conflict is causing problems. In this case, open the Terminal app by clicking cmd + space bar press and enter the term "Terminal" in the search. Alternatively, you can find the application under "Utilities". Then type in the term macerror and the error code to learn more.

Note: Some error codes are negative and have a minus sign. Remember to enter the minus symbol in the terminal app - macerror -43 is something else than macerror 43.

This app is not supported

You may have encountered the following Mac error message: You want to start an app, but the computer reports that the application's software is out of date. This can happen, for example, after updates to the operating system if the app does not support the latest version of macOS. In that case, you'll have to wait for the developer to update their application - or you can just download an alternative app. You could also install an older version of macOS that the app is compatible with. However, we would not recommend that to you. It makes more sense to keep your Mac's operating system up-to-date, as system updates often plug security holes and improve performance.

Safari web content quit unexpectedly? You can do that

Sometimes Apple's Safari browser closes suddenly without first selecting the X button. Most of the time, memory management is the cause, but other reasons are possible. The best way to empty your computer's cache is to switch on the developer menu under “Advanced” in the macOS settings and then first select “Developer” and then “Clear cache memory” in the top bar of your desktop.

Use the developer menu on your Mac to empty the memory cache.


  • If a prohibition sign or a question mark appears on your Mac display, there is a conflict with the startup disk: In the worst case, you will have to reinstall macOS to solve the problem.
  • If an app is not supported, you should check whether an update is available for the application or look for an alternative.
  • Use the Terminal app to find out what's behind certain Mac error codes.
  • If Safari quits unexpectedly, it helps to clear the cache.
  • If your Mac reports that its startup disk is almost full: Delete all files that you no longer need.

Do you know any other Mac error messages? Please share them with the community by leaving a comment here.