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What Does CHKDSK Really Do? From a Software Engineers View

imageI wish I had a nickel for every potential client that has called the shop and explained to me that they have a hard drive that is locking up (bad sectors usually) and it won’t populate the file system. As I am listening I hear things like

“I put it in an external enclosure and tried to recover the data”

“I took it to a friends’ house and had them take a look”

“I took it to my network admin at work and had them look at it”

All of the above quotes I have heard many times. I usually follow up with the question:

“So, have you done anything to recover the data yourself?”

I usually get a no. The next question I ask is:

“Did you run chkdsk’

To which they retort:

“Oh yeah! Windows ran that!”

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Which is essentially true. If a drive has bad sectors, and a component of the file system is affected then the operating system cannot resolve the cluster linking.

To the best of my knowledge this is how the operating system handles the mounting of a file system at boot up. The Master Boot Record boot code releases control to the OS boot record. That is loaded and then a pre-loader (NTLDR) is then executed. Each file system is then checked for continuity. It is during this check that an error may occur. If it does then chkdsk will run and try to resolve the file system anomaly. Each phase of chkdsk will validate a portion of the file system. As chkdsk looks at the clustering linking for each file it tries to read each cluster that is related to a single file. If the cluster cannot be read then chkdsk will truncate the file or make it zero bytes. The remaining valid cluster fragments are converted into files and give the file name FILE????.CHK. These files are actually the data that could not be mapped to the file system record. Sometimes the whole file is in one of these fragments and only takes a moment to rename the file to its original file name. In doing this you will have recovered your data with a minimal amount of pain.

Ultimately what you should do is make a clone of the drive onto one of the same size or larger and then recover your data from that drive. Using this technique will keep your system from locking up and allow for a more robust hard drive recovery.

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