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Why is the CHS is different from what is printed on the hard drive

Posted on 2016-10-16
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Last Modified: 2016-10-18
Why is the Cylinder-head-sector reported in a recovery software is different than from what is printed in the actual hard disk drive? (see pix below)

hd
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Question by:rayluvs
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Expert Comment

by:John Hurst
ID: 41845946
The label is for the total drive and the screen shot is for one of the partitions on the drive.
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by:CompProbSolv
CompProbSolv earned 125 total points
ID: 41845955
Hard drives have relied on translation of the actual CHS to different values for a very long time.  For whatever reason, what is seen by the OS may be very different from what is physically the case as seen in your particular instance.  If it were simply a matter of seeing one of the drive's partitions, you'd expect the cylinders to be less than the total but the heads and sectors/track to be consistent with the label.
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Assisted Solution

by:Dr. Klahn
Dr. Klahn earned 125 total points
ID: 41845967
CompProbSolv is correct.  The geometry of a drive is no longer of interest to the operating system since LBA addressing came along back in 1994 in 28-bit IDE.  CHS is a relic of MFM days when dumb controllers had to deal with cylinder, sector and head addresses and the drivers had to do LBA to CHS translation.

Further, CHS is a complete falsehood on modern drives, since variable sectoring of cylinders means that sector addresses are meaningless.
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Author Comment

by:rayluvs
ID: 41845968
Exactly, that is why we placed the question.

Prior placing the question we tried to understand it; for example, one of the links we read was in Wiki.  Still could grasp why the difference between the 2  values.  We also searched for calculators and found Media Size Calculator.  We placed in the calculator the values printed in the drive CHS '16383 16 63' we get approx 8.5 GB, but if we place the software CHS '121601 255 63', we get 1000.5 GB thus approx 1 TB (the drives size as per label); thus, more confusion.

If the software CHS 121601 255 63 is one partition of the drive, as stated by EE, then the other missing partition would make the drive over 1 TB.
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Author Comment

by:rayluvs
ID: 41845969
We sent the entry prior reading yours klahn.
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Author Comment

by:rayluvs
ID: 41845972
So in conclusion, CHS is wrong in both counts; what the software interpret and what is printed in the drive?
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Expert Comment

by:Dr. Klahn
ID: 41845979
Unless the drive is a very old drive without variable sectoring, that's the case.   The operating system uses LBA, not CHS to address the drive, and any drive using variable sectoring that presents CHS values is not being entirely truthful.
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Author Comment

by:rayluvs
ID: 41845997
The drive is from September 2012 as printed on the label; so it's not old.  We purchase a new drive a couple of weeks same model and date printed is of this year and also same CHS.
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by:John Hurst
ID: 41846000
Does it matter to you?  Not to me. New drives show their total capacity on the label and that balances within reason to what I get in space on the drive.
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Author Comment

by:rayluvs
ID: 41846016
We are just trying to understand.
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garycase earned 250 total points
ID: 41846018
"... We placed in the calculator the values printed in the drive CHS '16383 16 63' we get approx 8.5 GB, but if we place the software CHS '121601 255 63', we get 1000.5 GB thus approx 1 TB (the drives size as per label); thus, more confusion. " ==>  Note that the drive's plate shows BOTH CHS addressing with a size of 8455MB (the 8.5GB you noted) AND an LBA size of 1TB (1,953,525,168 sectors).     The utility you're using is showing the emulated CHS values, which map to the LBA addressing for the actual size of the drive.

Not sure why the CHS size is shown -- it's what would be used if this drive was installed in a system that still had the old 8.46GB (Int 13 interface) barrier.    It's been over a decade since any systems had this issue -- and I'm not aware of any current OS that still has this problem.     I suppose it's shown in case you were to install an old OS that would be sensitive to this restriction.
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Author Comment

by:rayluvs
ID: 41846038
Understood; the values displayed on the software is, as you state "emulated CHS values, which map to the LBA addressing for the actual size of the drive."  We can accept this as an answer as to the "why" both values are different.

Nevertheless, as an EE stated here "Does it matter to you?".  Well, technically maybe no since we are no experts on the matter and what will do it does matter??? Nothing.  Yet as concerned users that tries different software and purchases different software, it does raise concern as to the "why".  As all entries here from EE lead us to conclude that CHS is not important and we should pay no mind this value, though all software we have run on the unit displays the same CHS as stated in our question (see pix below)



We are no expert, believe us, we are just asking based on results we see.  And we want to say that EE has always helped us understand all these type of things; so thank you all in advance for your patience and sharing your knowledge.

Anyways, thanx all!
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Expert Comment

by:Steve Si
ID: 41848206
The maximum drive size for old systems which use only CHS and not LBA is 8GB (1023x255x63) because the MBR partition table and old BIOSes can only go up to these values (10 bits for cylinder, 8 bits for head, 6 for sector).
The drive label has "16383x16x63" which will be converted by an old BIOS to 1023x255x63.
Modern systems can use LBA values and the partition table was 'enhanced' to contain LBA values of up to 2TB. Even more modern partitioning systems use GPT which can go beyond 2TB,
So the two capacities on the HDD label are for old CHS BIOSes (rarely seen - max 8GB!) and LBA BIOSes.
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Author Comment

by:rayluvs
ID: 41849055
Thanx!!!
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