Open main menu

CHKDSK (short for "check disk") is a system tool in DOS, FlexOS,[1] 4690 OS,[2] OS/2 and Microsoft Windows. It verifies the file system integrity of a volume and fixes logical file system errors. It is similar to the fsck command in Unix and similar to Microsoft ScanDisk which co-existed with CHKDSK in Windows 9x and MS-DOS 6.x.

Chkdsk.exe in action on drive C: in Windows 10
Chkdsk.exe in action on drive C: in Windows 10
Original author(s)Tim Paterson
Developer(s)Seattle Computer Products, Digital Research, Microsoft, IBM, ReactOS Contributors
Initial release1981; 38 years ago (1981)
Operating systemDOS, FlexOS, 4690 OS, OS/2, Microsoft Windows, ReactOS




The CHKDSK command was first implemented by Tim Paterson and included in Seattle Computer Products 86-DOS.[3]


CHKDSK and UNDELETE in MS-DOS 5.0 have a bug which can corrupt data: If the file allocation table of a disk uses 256 sectors, running CHKDSK /F can cause data loss and running UNDELETE can cause unpredictable results. This normally affects disks with a capacity of approximately a multiple of 128 MB. This applies to CHKDSK.EXE and UNDELETE.EXE bearing a datestamp of April 9, 1991. This bug was fixed in MS-DOS 5.0a.[4]

Microsoft WindowsEdit

CHKDSK can be run from DOS prompt, Windows Explorer, Windows Command Prompt, Windows PowerShell or Recovery Console.[5]

On Windows NT operating systems, CHKDSK can also check the disk surface for bad sectors and mark them (in MS-DOS 6.x and Windows 9x, this is a task done by Microsoft ScanDisk). The Windows Server version of CHKDSK is RAID-aware and can fully recover data in bad sectors of a disk in a RAID-1 or RAID-5 array if other disks in the set are intact.[6]

On Windows NT family, a standard CHKDSK scan consists of three phases of testing file metadata. It looks for errors but does not fix them unless it is explicitly ordered to do so. The same applies to surface scan—this test, which could be extremely time-consuming on large or low-performance disks, is not carried out unless explicitly requested. CHKDSK requires exclusive write access to the volume to perform repairs.[7][8]

Because of the exclusive access requirement and the time-consuming nature of CHKDSK operation, Windows Vista implemented a new file system health model in which the operating system fixes errors on the volumes as it encounters them. In the event that the problem is grave and a full scan is required, Action Center notifies the user to take the volume offline at the first convenience.[9]

Windows Vista and Windows Server 2008 added self-healing ability, turned on by default, in addition to providing the CHKDSK command. It detects physical file system errors and silently fixes them on the fly. Thus, many problems previously discovered on running CHKDSK never appear. It is administered by fsutil repair command.[10][11]

The alleged Windows 7 bugEdit

Before the release of Windows 7, InfoWorld reported an alleged memory leak in CHKDSK; according to the report, the chkdsk /r command would cause the memory consumption to reach the maximum and the system to crash. Randall C. Kennedy of InfoWorld attributed the original report to "various Web sources" and said that in his tests, the memory consumption reached above 90%, although he did not experience a crash. Nevertheless, Kennedy took the memory consumption for a critical bug that would derail Windows 7's launch and chastised Microsoft.[12] Tom Warren of Neowin dismissed Kennedy's assessment of the alleged leak's significance.[13] Steven Sinofsky of Microsoft also responded that Microsoft could not reproduce a crash either but that the massive memory consumption was by design, to improve performance, and not a leak. Ed Bott of ZDNet also reviewed the claim with his own tests and observed that no crash would occur. Noting that chkdsk /r, by design, does not work on the system drive while Windows is online, Bott concluded "it’s arguably a feature, not a bug, and the likelihood that you’ll ever crash a system this way is very, very small and completely avoidable."[14]


The ReactOS implementation is based on a free clone developed by Mark Russinovich for Sysinternals in 1998.[15]

See alsoEdit


  1. ^
  2. ^
  3. ^ 86-DOS User's Manual - Version 0.3
  4. ^ "When Not to Use MS-DOS 5.0 CHKDSK and UNDELETE Commands". Support (1.1 ed.). Microsoft. 16 November 2006.
  5. ^ "Description of the Windows XP Recovery Console for advanced users". Support (8.0 ed.). Microsoft. 11 July 2013. Retrieved 4 September 2014.
  6. ^ Holm, Dan; Thomas, Orin (2003). "11-3: Maintaining Disk Storage Volumes". Managing and Maintaining a Microsoft Windows Server 2003 Environment. Microsoft Press. p. 11.25. ISBN 9780735614376.
  7. ^ "An explanation of the new /C and /I Switches that are available to use with Chkdsk.exe". Microsoft Support. Microsoft. 1 December 2007. Archived from the original on 24 October 2014. Retrieved 25 January 2019.
  8. ^ "Troubleshooting Disks and File Systems". Windows XP Professional Resource Kit. Microsoft. 3 November 2005. Retrieved 4 September 2014.
  9. ^ Bangalore, Kiran (10 May 2012). Sinofsky, Steven (ed.). "Redesigning chkdsk and the new NTFS health model". Building Windows 8. Microsoft.
  10. ^ Chen, Ken (4 August 2014). "NTFS Self-Healing is An Overlooked but Useful Feature in Windows 7". Next of Windows.
  11. ^ Pawar, Amit (13 February 2008). "Self-healing NTFS in Windows Server 2008 and Windows Vista". Amit Pawar - Infrastructure blog. Microsoft.
  12. ^ Kennedy, Randall C. (5 August 2009). "Critical Windows 7 bug risks derailing product launch". InfoWorld. IDG. Retrieved 19 February 2015.
  13. ^ Warren, Tom (5 August 2009). "Windows 7 RTM contains a rather nasty chkdsk bug". Neowin. Retrieved 19 February 2015.
  14. ^ Bott, Ed (5 August 2009). "A killer Windows 7 bug? Sorry, no". ZDNet. CBS Interactive.
  15. ^

Further readingEdit

External linksEdit