What a disk check does and when you need one

A disk check scans your hard drive or solid-state drive for errors — bad sectors, corrupted files, and problems with how Windows or macOS keeps track of what's stored where. Running one won't speed up a slow computer or free up space, but it can catch problems before they cause data loss or system crashes.

You should run a disk check if your computer freezes randomly, takes a long time to start up, or if you've had an unexpected shutdown or power loss. You might also run one after installing new hardware or if you see error messages about disk problems. On older drives, a disk check every few months is reasonable preventive maintenance.

The process takes time — anywhere from a few minutes to several hours depending on drive size and how many errors the tool finds. Plan to run it when you don't need your computer for a while, or schedule it to run at the next startup.

Key Takeaways

  • Windows uses the Check Disk tool (chkdsk), which you can run from the command prompt or schedule to run at startup.
  • macOS uses Disk Utility, found in Applications > Utilities, and the First Aid tool within it scans and repairs drive errors.
  • A full disk check with repairs requires administrator access and usually takes longer than a quick scan.
  • If your drive has serious errors, the tool may recommend backing up your files and replacing the drive.

Running Check Disk on Windows

Open the Start menu and type cmd. Right-click on Command Prompt and select "Run as administrator" — the tool needs full system access to scan and repair. If you see a User Account Control prompt, click Yes.

At the command prompt, type chkdsk C: /F /R and press Enter. The C: refers to your main drive (use a different letter if you're checking a different drive). The /F flag tells it to fix errors it finds, and /R tells it to locate bad sectors and recover readable information. Windows will ask if you want to schedule the check to run at the next startup — type Y for yes. Restart your computer when prompted.

At startup, you'll see a black screen with a progress bar. The scan runs before Windows loads, so you can't use your computer during this time. If the drive has many errors, the process can take several hours. When it finishes, Windows starts normally and you can check the results by opening Event Viewer (search for it in the Start menu), navigating to Windows Logs > System, and looking for entries labeled "Chkdsk".

Running Disk Utility on macOS

Open Finder, go to Applications, then Utilities, and double-click Disk Utility. On the left side, you'll see a list of drives. Click the drive you want to check — usually the one labeled "Macintosh HD" or similar for your main drive.

Click the First Aid button at the top. macOS will ask for your password because the tool needs administrator access. Enter it and click OK. The scan begins immediately and shows a progress bar. First Aid checks the drive's file system and repairs errors automatically if it finds them.

When the scan finishes, you'll see a message saying the drive is okay or describing any problems found. If First Aid reports serious errors it cannot repair, back up your files to an external drive and consider having the drive replaced. If it reports minor errors that it fixed, your drive should work normally going forward.

Understanding what the tools report

Both Windows and macOS disk check tools report findings in plain language. A message saying "no errors found" or "drive is okay" means your drive is healthy. A message about "fixed errors" means the tool found and corrected problems — your drive should work normally, but monitor it over the next few weeks for new issues.

If the tool reports "bad sectors" or "unrecoverable errors," it means parts of the drive are physically damaged and cannot be repaired by software. The tool may have recovered data from those sectors or marked them as unusable so your system won't try to write to them again. This is a sign the drive is aging and you should back up important files soon and plan to replace it within the next few months.

A report of "file system errors" means the drive's organizational structure is corrupted — the tool fixed this, but if errors reappear after another check, the drive may be failing. Contact the drive manufacturer's support or a local repair shop if you're unsure whether to replace it.

Checking external drives and USB devices

You can run a disk check on external hard drives, USB flash drives, and SD cards the same way you check your main drive. On Windows, use the same chkdsk command but replace C: with the letter of your external drive (you can find this in File Explorer). On macOS, select the external drive in Disk Utility and click First Aid.

External drives often develop errors from being unplugged while in use or from physical damage. If a disk check finds errors on an external drive you use for backups, don't rely on it for important files until you've run the check again and confirmed no new errors appear.

What to do if the disk check won't run or gets stuck

If you schedule a disk check on Windows and it doesn't run at startup, restart your computer and watch the screen carefully — sometimes the prompt to begin the scan appears for only a few seconds. Press any key when you see it.

If the scan starts but appears to be stuck on the same percentage for more than an hour, your drive may have serious hardware problems. Let it run for at least two hours before assuming it's frozen — large drives with many errors can take this long. If it's still stuck after two hours, force a shutdown by holding the power button for 10 seconds, then take the computer to a repair shop.

On macOS, if First Aid won't start or reports that it cannot repair the drive, your drive may have hardware failure. Back up your files immediately if you haven't already, and plan to replace the drive soon.

Frequently Asked Questions

Will a disk check delete my files?

No. A disk check scans for errors and repairs the file system structure, but it doesn't delete your files. If the tool finds corrupted files, it may quarantine them or move them to a recovery folder, but your data remains on the drive.

How often should I run a disk check?

For most people, running a disk check once every few months is reasonable. If you notice problems like freezing or slow startup, run one immediately. If a check finds errors, run another one a week later to see if new errors appear — repeated errors suggest the drive is failing.

Can I use my computer while a disk check is running?

Not during a full scan with repairs. On Windows, the check runs at startup before the operating system loads. On macOS, you can use your computer while First Aid runs, but performance will be slower. For the most accurate results, don't use the computer during the scan.

What does "bad sectors" mean?

A sector is a small physical area on your drive where data is stored. A bad sector is one that's damaged and can't reliably hold data. The disk check tool marks bad sectors so your system won't try to use them, but it's a sign your drive is aging and you should plan to replace it.

Do I need to buy software to check my disk?

No. Windows Check Disk and macOS Disk Utility are built into the operating system and free to use. Third-party tools exist, but the built-in tools are sufficient for most situations.