A reader asks: Why does Disk Defrag fail, despite plenty of room?

Reader Oystein Ravner asks (via the CONTACT link):

Hi Fred! I am glad you are back! Problem: W7 Disk Defragmenter. System Disk is successfully analyzed, but with result 41% fragmented. Clicking ‘Defragment Disk’ leads to nothing. System Disk has capacity 100MB, free 70%, status Healthy. Why is there no defrag action when Microsoft advises such action when the fragment factor is higher than 10%? With kind regards!

That’s definitely odd!

You’ve already checked for the most common reason why a defragmentation may fail: insufficient free space. Generally, defragmentation needs around 15% of disk space elbow room to work. Your 70% free is way over that!

So, the problem likely lies elsewhere.

I suspect a disk error; most likely some logical (soft) error that’s tripping up the defragger app.

You can easily check your drive’s logical health with Windows’ built in error-checking applet; it’s close by the defrag tool you’re probably already using:

In File Explorer, right-click the drive that you want to check and select Properties/Tools/Error-checking/Check now. Let the disk check run to completion, and allow it to correct any errors it finds.

Alternatively, you can use the command-line CHKDSK (“Check Disk”); open an admin-level command window and enter:

chkdsk /f

(The /f switch tells chkdsk to fix any errors it finds.)

If chkdsk won’t run, take heart: the same error that’s blocking chkdsk is probably what’s blocking defrag. Fix one, and you’ll probably fix the other.

The #1 reason why chkdsk won’t run is that some other app — often, an anti-virus/anti-malware or other kind of disk-monitoring/-access tool is also running, and getting in the way.

To see if this is the case, disconnect your PC from the internet (for safety), and temporarily deactivate your antivirus (or whatever) tool. With nothing else contending for disk access, both chkdsk and defrag should now work.

If necessary, you can even boot to Safe Mode, and run the error check from there: safe mode prevents most third-party apps from running, and should give chkdsk and defrag clear sailing. (Microsoft info for Win7 Safe Mode.)

If even Safe Mode doesn’t help, check your disk’s physical health using its built-in Self-Monitoring, Analysis, and Reporting Technology (aka SMART) subsystem; there are many free SMART apps to assist you (examples) and they may be able to provide more detail on problems than Windows can.

If the drive’s logical and physical health check out, but you still can’t run defrag, you still have options left.

First, even though your disk is showing 70% free, it’s possible (not likely, but possible) that there’s some huge file (or files) too large to be moved because they’re larger than the largest-available unfragmented area; or are locked by permissions that defrag can’t get around. For example, your system may be carrying copies of many old system files; these backup copies are made automatically by Windows Update. The Update files can be huge, and they’re protected from normal file access.

But you can remove those files via Windows’ “Clean up system files” option.

To clean up system files in Win7: In File Explorer, right-click the drive that you want to check and select Properties/General/Disk Cleanup. The app will churn for a bit as it scans your drive for unneeded files.

When the Disk Cleanup dialog appears, click the Clean up system files button (see Figure 1).

Reselect the system drive (again, usually C:). Click OK. The app will churn once more as it rescans your drive for unneeded system-level files, including obsolete Updates. A new dialog box will open.

Disk Cleanup will then display an expanded list of files available for deletion. Make sure that Windows Update Cleanup is checked along with any other obsolete system-level files you want to remove (see Figure 2). Click OK when you’ve made your selections. (Note: If your system doesn’t currently contain any unneeded update files — such as in new or recently cleaned systems — the Windows Update Cleanup item won’t appear.)

(Windows 10 users note: The classic Win7-style Disk Cleanup tool is going away. See Say goodbye to Windows’ classic Disk Cleanup Tool for info on the new Win10 “Free up space” option, which includes a Windows Update cleanup.)

It’s hard to imagine all the above still not getting things working, but if that’s where you find yourself, you’re down to the last options: Something must be majorly wrong with your copy of defrag, or the OS itself.

To bypass a mangled defrag app, you can try a third-party defrag tool. There are a million of ‘em, free and paid: examples. One of those might be able to power past whatever’s messing up the native Windows defrag.

Still stuck? All that’s left is a reinstall, or a no-reformat reinstall, of your OS. For more info, see this related article.

But odds are, one of the faster, simpler fixes described earlier should get defrag going again!

(See comments, below, for additional information.)

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