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Windows 7: CHKDSK zeroes out bad sectors

13 Apr 2012   #1

Windows 7 Ultimate x64
CHKDSK zeroes out bad sectors

While using my laptop to load some old training DVDs for archiving, I discovered evidence of a bad sector on my laptop's hard drive (Seagate Momentus 7200.4 SATA 500GB): upon investigating file compare errors, I found one fairly large file (464MB) where, after several tries, everything matched perfectly except the last 16 bytes which displayed three or four variations on the hard drive.

While running Check Disk (at Windows 7 Ultimate x64 boot-up, initiated from Windows Explorer with the option "Scan for and attempt recovery of bad sectors"), CHKDSK reported a single bad sector -- in the file in question -- and claimed to have fixed it. So far, so good.

After running file compares again, though, I found that CHKDSK had zeroed out the last 32768 bytes. I expected all but the last 16 bytes of the file to be correct; why was any of it zeroed out?

The last time I had such detailed experience with CHKDSK and live data involved MS-DOS and floppy disks, and I'm pretty sure CHKDSK really did recover most or all of the data in question. So this was a surprise. Why couldn't Windows 7's CHKDSK succeed at "attempt[ing] recovery of bad sectors" and at least read what ordinary utilities could read?

At the very least, this is motivation to be diligent about backups!

My System SpecsSystem Spec

13 Apr 2012   #2

Windows 8.1 Pro (x64)

Keyword "attempt" recovery of bad sectors. Attempt, not guaranteed, attempt. And as it is CHKDSK does not remove data it saves what it can and moves it, if it couldn't move those "32768 bytes" you are concerned about its probably you have bigger issues with your drive then you thought.
My System SpecsSystem Spec
13 Apr 2012   #3

Windows 7 Pro with SP1 32bit

You are most welcome to Seven Forums InquiringMind.

Now that a bad sector has developed on the laptop's hard drive, you should keep a close watch on it. This is the first sign that it is soon going to need a change.
My System SpecsSystem Spec

13 Apr 2012   #4

Windows 7 Ultimate x64

I certainly plan on keeping watch on my hard drive, although the only problem identified so far was this single bad cluster. And I am not worried about the file, which was only a copy to begin with.

My question is still about CHKDSK. If it had "attempted" recovery of the data at least as well as three other utilities I used multiple times to test and investigate the problem, CHKDSK would have succeeded in reading all but 16 bytes -- and those most likely would not have ended up as zeros. The mere fact that the last cluster was entirely zeroed out rather than mostly the correct original data (or even garbage if there were additional read errors because of the recovery attempt), suggests to me that CHKDSK is NOT attempting to recover the data. If not, why make the claim?
My System SpecsSystem Spec
13 Apr 2012   #5

Windows 8.1 Pro (x64)

Why make the claim? Because it does recover data that it can. Again it comes back to "attempt" recovery of bad sectors. Read: Windows Internals, 5th Edition Chapter 11 Page 985, NTFS Bad-Cluster Recovery.

Also, a cluster by default is about 4096 Bytes, When a sector is deemed to be bad, it marks the whole cluster as bad. Clusters are the smallest unit file systems work with. (4K clusters hold about 8 sectors.)
My System SpecsSystem Spec
15 Apr 2012   #6

Microsoft Community Contributor Award Recipient

Windows 7 Home Premium x64 SP1

Suggest you download Seatools, and create a bootable disc to run on boot. Seatools can often repair bad sectors.

SeaTools for DOS | Seagate

| Seagate

A Guy
My System SpecsSystem Spec

 CHKDSK zeroes out bad sectors

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