What Check Disk Does and When You Need It

Check Disk (also called chkdsk on Windows or Disk Utility on Mac) scans your hard drive for errors and can fix some of them automatically. It finds bad sectors — small areas of the disk that no longer store data reliably — and logical errors where the file system thinks a file exists but the actual data is missing or corrupted. If your computer won't start, files open as garbage, or you see error messages about corrupted files, Check Disk can often repair the damage.

You do not need to run Check Disk on a schedule. Run it only when something goes wrong: after an unexpected shutdown, when a file won't open, when Windows shows a blue screen error, or when your disk makes clicking sounds. Running it unnecessarily takes hours and puts wear on an aging drive.

Check Disk works differently depending on whether the drive is your boot drive (the one Windows or Mac runs from) or a separate external or secondary drive. The boot drive requires a restart and runs before the operating system loads. Other drives can be checked while you work.

Key Takeaways

  • Check Disk on Windows requires administrator access and works best when run from the command prompt with the /F flag to fix errors automatically.
  • If the drive is your boot drive, Windows will schedule the scan to run at the next restart and complete before the operating system loads.
  • External drives and secondary internal drives can be checked without restarting by right-clicking the drive and selecting Properties, then Tools.
  • A full scan with repair can take several hours on a large drive, so run it when you do not need the computer.
  • If Check Disk finds bad sectors it cannot repair, the drive is failing and you should back up your files when ready and plan to replace it.

Running Check Disk on Your Boot Drive in Windows

The drive Windows runs from cannot be checked while the operating system is active, so Windows schedules the scan to run at the next restart. Open the Start menu, type cmd, right-click Command Prompt, and select Run as Administrator. Type this command exactly:

chkdsk C: /F /R

Replace C: with the letter of your boot drive if it is different. The /F flag tells Check Disk to fix errors it finds. The /R flag tells it to locate bad sectors and recover readable information. Press Enter. Windows will ask if you want to schedule the scan at the next restart. Type Y and press Enter. Restart your computer when you are ready — the scan will run before Windows loads, which takes 15 minutes to several hours depending on drive size and how many errors exist.

Do not turn off the computer or unplug it while the scan runs. If the scan finds errors it cannot fix, it will tell you at the end. Write down any error messages before restarting again, because you will need that information to decide whether the drive can be repaired or needs replacement.

Running Check Disk on External Drives and Secondary Drives

External drives and secondary internal drives do not require a restart. Plug in the external drive or make sure the secondary drive is connected. Open File Explorer, right-click the drive, and select Properties. Click the Tools tab. Under Error Checking, click Check. Windows will ask if you want to scan and repair the drive. Click Scan and Repair.

The scan runs in the background while you work, though the drive will be slower. Do not unplug the drive while the scan is running. When it finishes, Windows will show a summary of what it found and fixed. If it found bad sectors, note how many — a few is normal on an old drive, but dozens or hundreds means the drive is failing.

Running Check Disk on a Mac

On Mac, the tool is called Disk Utility. Plug in the external drive or make sure the secondary drive is connected. Open Applications, go to Utilities, and click Disk Utility. Select the drive from the list on the left. Click the First Aid button at the top. Click Run. Disk Utility will scan and repair the drive automatically.

The scan takes 10 minutes to an hour depending on drive size. You can use your Mac while it runs. When it finishes, Disk Utility shows whether it found and fixed errors. If the drive is your boot drive (the one Mac runs from), you may need to restart into Recovery Mode first — Disk Utility will tell you if that is necessary.

What to Do If Check Disk Finds Uncorrectable Errors

If Check Disk reports that it found bad sectors or errors it could not repair, your drive is developing hardware failure. This does not mean the drive will stop working tomorrow, but it means the failure has started. Back up your important files to an external drive or cloud storage when ready. Do not wait.

After you back up, you have two choices: continue using the drive and monitor it for more errors, or replace it now. If the drive is old (five years or more), replacement is usually the safer choice because the failure will likely accelerate. If the drive is newer and the error count is small, you can continue using it while you save money for a replacement, but check it again in a month.

Do not rely on a drive with uncorrectable errors for anything you cannot afford to lose. If it is your boot drive, consider replacing it before it fails completely and leaves you unable to start your computer.

Understanding Check Disk Results and Error Messages

When Check Disk finishes, it shows a summary. "No errors found" means the drive is healthy. "Errors found and fixed" means Check Disk repaired the problems — your drive should work normally now. "Errors found but not fixed" means the drive has damage that Check Disk cannot repair, which indicates hardware failure.

Some error messages mention "bad clusters" or "bad sectors." A cluster is a unit of disk space; a sector is a physical location on the disk. Both terms mean the same thing in practical terms: that part of the drive no longer works. A small number of bad sectors is not unusual on a drive that is several years old. Hundreds of bad sectors means the drive is failing.

If you see "The type of the file system is NTFS" or "RAW," the drive's file system is corrupted beyond what Check Disk can fix. This usually requires professional data recovery or replacement of the drive.

How Long Check Disk Takes and Why

A quick scan with no repair takes 10 to 30 minutes on a 1 TB drive. A full scan with repair (/R flag) takes 2 to 8 hours on the same drive because it reads every sector and tests it. Larger drives take longer — a 4 TB drive might take 12 to 24 hours. The exact time depends on how many errors exist and how fast your drive reads.

Run Check Disk when you do not need the computer. If it is your boot drive, schedule it for a time when you can leave the computer running overnight. If it is an external drive, connect it before you go to bed and let it scan while you sleep. Do not interrupt the scan by unplugging the drive or restarting the computer — stopping it partway through can make corruption worse.

Frequently Asked Questions

Can I use my computer while Check Disk runs on an external drive?

Yes, but the external drive will be slow or unavailable while the scan runs. You can use other programs and drives normally. Avoid opening files on the drive being scanned, because that can interfere with the repair process.

What does the /R flag do that /F does not?

The /F flag fixes logical errors in the file system — places where the system thinks a file exists but the data is wrong. The /R flag finds bad sectors and tries to recover any readable data from them. Use /R if the drive has physical damage or if files are corrupted. Use /F alone if you only see file system errors.

Will Check Disk delete my files?

Check Disk does not delete files intentionally. If it finds a file that is corrupted beyond repair, it may move the unreadable parts to a folder called Lost and Found, but the file itself remains. In rare cases, if the file system is severely damaged, some files may become inaccessible, but this is a result of the damage, not the scan.

Do I need to run Check Disk if Windows offers to repair the drive at startup?

Windows sometimes offers to repair the drive automatically after an unexpected shutdown. Let it do so — this is Check Disk running in the background. You do not need to run it again unless the problem happens again or you see error messages about corrupted files.

What should I do if Check Disk keeps finding the same errors?

If the same errors appear after multiple scans, the drive has permanent hardware damage that cannot be repaired. Back up your files and plan to replace the drive. Continuing to use it risks losing data.