Why is it a bad sec...
 
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Why is it a bad sector, not a bad byte or bad bit?

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(@yunus)
Estimable Member
Joined: 17 years ago
Posts: 178
Topic starter  

As we know, a sector has 512 byte, so it can store 512 characters. And a sector is the smallest unit that could be addressed. However, this situation brings up some questions in my mind.

When there is bad sector on a hard drive, the head can not read that sector and we lose all 512 characters. What I want to find out is Does the head read one sector as a whole, like a snapshot, or does it read each bit (or byte) in a sector?

And if yes, why do we say bad sector and not say 'bad byte' or 'bad bit' instead of if, and be more specific about what is actualy bad, and possbibly lose less amount data?

Thanks


   
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PaulSanderson
(@paulsanderson)
Honorable Member
Joined: 19 years ago
Posts: 651
 

The disk is read a sector at a time. Each sector has an error correction check that is carried out across all 512 bytes, the correction algorithm can correct small amounts of data but because of the mathematics behind the code the controller doesn't know which bytes are corrupt when it can't fix them, it just knows that some of the bytes are corrupt. Hence the whole sector is flagged as dodgy.


   
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(@rarosalion)
Eminent Member
Joined: 17 years ago
Posts: 28
 

Not a forensic tool, but Spinrite (from grc.com) apparently will read the "bad sector" multiple times in an attempt to recover as much data from it as possible before allowing the drive to finally re-map the sector to another area of the disk.


   
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markg43
(@markg43)
Trusted Member
Joined: 18 years ago
Posts: 77
 

Before you run spinrite, check out this post by Scott Moulton of MyHardDriveDied.com

http//www.myharddrivedied.com/weblog/why_spinrite_is_not_on_my_d.html

Careful about using it, as Scott says it re-writes the recovered data back onto the drive with the bad sectors. This may or may not be bad for the drive. Just be aware.

MarkG


   
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(@mscotgrove)
Prominent Member
Joined: 17 years ago
Posts: 940
 

I think you will find that hard disks are not as simple as floppy disks. Sectors can be stored is larger blocks. Because the media does not have to read in other drives, the drive manufacturer can use any logic they want. However, logically, the smallest length of data is normally the 512 byte sector.

As Paul says, error correction often works with a single bit, or few bits of error, so you will never see them. Any more error bits, and no data is reliable for that sector.


   
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(@ravalert)
Eminent Member
Joined: 20 years ago
Posts: 21
 

TS seemed to have answered the question himself?
"a sector is the smallest unit that could be addressed"

Unless there is new technology emerging (or maybe I have no idea about its presence), we may not be able to access good bytes in the bad sector. ??


   
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