A Windows 10 update rolled out by Microsoft contained a buggy version of chkdsk that damaged the file system on some PCs and made Windows fail to boot.
The updates that included the fault are KB4586853 and KB4592438. Microsoft's notes on these updates now incorporate a warning: "A small number of devices that have installed this update have reported that when running chkdsk /f, their file system might get damaged and the device might not boot."
The notes further reveal: "This issue is resolved and should now be prevented automatically on non-managed devices," meaning PCs that are not enterprise-managed. On managed PCs Microsoft recommended a group policy setting that rolls back the faulty update. If there are devices that have already hit the issue, Microsoft has listed troubleshooting steps which it says should fix the problem.
The chkdsk utility itself is not listed in the files that are patched by these updates, suggesting that the problem is with other system files called by chkdsk.
The problem was described by users in a German forum, as mentioned on a site run by Windows expert Günter Born here. An administrator for a school IT system ran chkdsk after installing updates, rendering seven PCs unbootable. All the affected systems had SATA SSD drives, causing speculation that it is related to SSD issues. "The /f option of chkdsk probably destroyed the NTFS file system," said the report. "Further analysis of the RAW partition with chkdsk in offline mode revealed errors with a corrupted 'file 9' and an error in the BITMAP attribute of the Master File Table. These could be corrected with chkdsk in offline mode."
Earlier this month, Microsoft said that KB4586853 fixed a compatibility issue with Thunderbolt NVMe SSDs. It is always possible that fixing one problem introduced another.
Microsoft does not publish full details of what is fixed in its updates, only noting key changes or highlights, and the list of changed files. Administrators have to put up with generic descriptions like "this security update includes quality improvements." Although it is always possible that an update will break a working system, the security risks of not updating are greater in most cases.
The original chkdsk goes back to the earliest days of DOS. It can be used either just to scan, or to scan and attempt to fix file system errors. In Windows 10, chkdsk may run automatically at boot if file system errors are detected.
The success of the Windows-as-a-Service concept depends on reliable updates, though given the huge diversity of PC hardware occasional issues like this one are unsurprising. The timing and content of Windows updates is complex and can be configured in various ways, as described here. ®