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Stages 1-4 are mostly read only unless errors are found, but I'm more curious about stage 5, where it checks unused space for issues. How does that determine if a sector is bad? Does it write to the unused space, then try to read it back? Or something else?

Reason for asking: I have a WD Blue 1TB HDD that I bought probably in the early 2010s that I use to store data on. Nothing important that I can't just redownload from various locations or already have mirrored elsewhere, so I don't care about if the entire thing fails. It's just the biggest and fastest HDD I've got, so use it as my "primary" data storage device. I recently had an issue where I couldn't play back a video file. After running chkdsk /r on the drive, it found a single bad cluster worth 4KB. Not the end of the world, just 1TB minus 4KB of available storage. Redownloaded the video file, and done.

So I figured I'd set up a monthly chkdsk /r using simple batch files and Windows' built in Task Scheduler, so I could monitor if it gets any worse. While I was at it, I figured I'd set it to run on all my drives, and in parallel, so everything could be done while I was sleeping. All of my drives (except C: where Windows is installed) are HDDs salvaged from older PCs as the rest of the PC became too old to reasonably function, or other parts of the PC failed.

But then I got to thinking about Stage 5, where it checks the unused space for issues. If it does write to that empty space, then try to read it back, that would mean two things. One, that attempting to recover any deleted files using Recuva or similar (if needed) would be more difficult as the data is overwritten, and two, that chkdsk /r should not be run on SSDs as stage 5 would wear out the SSD faster.

Is my theory correct? I tried looking in Task Manager and Resource Monitor to see if it was reading or writing to the drive, but I guess those only report file accesses, and since there is no file being accessed, the tools report the drive is idle.

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  • Side note: if a disk is starting to show bad sectors there are usually more to follow. And re: nothing important... it's not just if you can find everything back, also consider the time you'll spend downloading and sorting everything. A new 1TB would be about €30, and if you go even bigger the price per TB goes down even more.
    – MiG
    Commented Jan 13, 2022 at 8:52
  • And SSDs manage their own health and wear out faster with excessive read/write operations, so it's advisable to stick with the manufacturer's monitoring software (Magician for Samsung for example).
    – MiG
    Commented Jan 13, 2022 at 8:54

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[... But then I got to thinking about Stage 5, where it checks the unused space for issues. If it does write to that empty space, then try to read it back, that would mean two things. One, that attempting to recover any deleted files using Recuva or similar (if needed) would be more difficult as the data is overwritten, and two, that chkdsk /r should not be run on SSDs as stage 5 would wear out the SSD faster.

Is my theory correct? ...]

No. As writing to a broken sector would trigger writing to a spare sector as long there are still some spares left, writing stuff into unused clusters and trying to reread it is not the proper strategy. Only reading is required because the underlying firmware of the drive can't influence the result of the operation as opposed to a write transaction.

In case you don't believe me and people pretending that this not a "proper answer" read here:

https://web.archive.org/web/20080106183532/http://technet.microsoft.com/en-us/library/bb457122.aspx

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