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This is part of output of fdisk -l

Disk /dev/sdb: 1000.2 GB, 1000204886016 bytes
255 heads, 63 sectors/track, 121601 cylinders, total 1953525168 sectors
Units = sectors of 1 * 512 = 512 bytes
Sector size (logical/physical): 512 bytes / 4096 bytes
I/O size (minimum/optimal): 4096 bytes / 4096 bytes
Disk identifier: 0x000140bd

   Device Boot      Start         End      Blocks   Id  System
/dev/sdb1            2048   167774207    83886080    7  HPFS/NTFS/exFAT
/dev/sdb2       167774208  1740638207   786432000    7  HPFS/NTFS/exFAT
/dev/sdb3   *  1740638208  1935673343    97517568   83  Linux
/dev/sdb4      1935673344  1952448511     8387584   82  Linux swap / Solaris

But when I multiply 255*63*121601 i get 1953520065 sectors which is less than 1953525168 sectirs reported by fdisk. The difference is 5103 sectors.

I know that the number of heads, sectors/track and cylinders reported does'n probably refer to real hardware properties and that is was somehow calculated for backward compatibility. But I still wonder what did hapen with those 5103 sectors? Can I use them?

Later I wanna move filesystems, do some changes, change partition table and use combination of GPT and MBR. That is why I want to know everything about sectors on my disk. My changes will include backing up with dd utility before start and after completing changes. I want to back up GPT sectors. Even those ones which should reside in last 34 sectors of HDD. But now I wonder where last 34 sectors are. On the reported end or on the place I calculated?

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1 Answer

CHS is a very old way to address sectors on a hard drive and usually limits the total number of sectors addressable to numbers divisible by the Cylinder and Head count. So in this case it will be a number divisible by 63 * 255 or 16065. The sectors on the drive in excess of the closest such number won't be used.

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