Formatted and Unformatted Storage Capacity of hard disk drive primary secondary logical partitions table partition high level low level formatting

 
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Formatted and Unformatted Storage Capacity

The total storage of a hard disk depends on, if you are looking at the formatted or unformatted capacity. Some portion of the space on a hard disk is taken up by the formatting information that marks the start and end of sectors, ECC (Error Correction Codes) and other overhead information. For this reason, the difference can be quite significant.

Older drives that were typically low-level formatted by the user often had their size listed in terms of unformatted capacity. For example, take the Seagate ST-412, the first drive used on the original IBM PC/XT in the early 1980s. The "12" in this model number refers to the drive's unformatted capacity of 12.76 MB. Formatted, it is actually a 10.65 MB drive.

Unformatted capacity of a hard disk is generally 19% (19 percent) higher than its formatted capacity. Since nobody can use a drive that is unformatted, the only thing that matters is the formatted capacity and therefore modern drives are always low-level formatted by the manufacturers.

The capacity of a hard disk can be expressed in the following four ways:

  • Formatted capacity in millions of bytes
  • Formatted capacity in megabytes
  • Unformatted capacity in millions of bytes
  • Unformatted capacity in megabytes
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Now if I have a hard disk with C–H–S = 1024*63*63 (It means that the disk has number of cylinder = 1024, number of heads or sides = 63 number of sectors per track = 63) and every sector having 512 bytes. The formula that will calculate the size of the disk is as follows:

Total Size of the Disk (Bytes) = (Cylinders) X (Heads) X (Sectors) X (Bytes per Sector)

 

By this formula when we calculate the size of the given hard disk in bytes, it will be

= 1024 X 63 X 63 X 512
= 2080899072 Bytes

Now if I calculate the Size of my disk in millions of bytes, it will be approximately

= 2080.899072
~ 2081 millions of byte

Traditionally size in millions of byte is represented by M. Therefore the size of my disk in millions of bytes is approximately 2081 M.

But when I tell the capacity of my hard disk in Megabytes, It will be approximately 1985 and will be written as 1985 Meg.

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