Sunday, September 19, 2010

Why my drive shows less space than expected?

Have you ever wondered when you plug in your new hard disc drive and it shows less free space then expected? This ain’t the question you thought for the first time. If you google this query, you’ll find over 3.7 million results, and this post that you are reading is probably the first one. so you are in-a-way, landed on the right page Smile

Well that’s not very obnoxious. Its pretty simple, rather a mere numbers game.

In order to determining the disc drive capacity you have to count on the operating system. This is mainly because of the file system they user. When dealing with systems based on Windows and Mac, you will often see both binary measurements and decimal measurements of the capacity of your drive. In both cases, the capacity of a drive is measured by using the total number of bytes available on it. As long as the drive displays the correct number of bytes (approximate), you are getting the full capacity of that drive.

Decimal vs. Binary:
First up, the hard drive manufacturers define a megabyte as 1,000,000 bytes and a gigabyte as 1,000,000,000 bytes for the obvious sake of simplicity and consistency. This is industry standard is basically a decimal (base 10) measurement. However, certain systems like BIOSs, FDISK and Windows define specifically a megabyte (MB) as 1,048,576 bytes and a gigabyte (GB) as 1,073,741,824 bytes. Mac and most other systems also use these values. These are binary (base 2) measurements.

To Determine Decimal Capacity:
A decimal capacity is determined by dividing the total number of bytes, by the number of bytes per gigabyte (1,000,000,000 using base 10).


To Determine Binary Capacity:
A binary capacity is determined by dividing the total number of bytes, by the number of bytes per gigabyte (1,073,741,824 using base 2).

This is the main reason why different utilities actually report different capacities for the same drive. The number of bytes is the same, but a different number of bytes is used to make an MB and a GB.


Important: If you are dealing with drive sizes smaller than the Approximate Binary Capacity of your Drive Size, then you may be dealing with either a BIOS limitation, or a Windows drive size limitation. For more information, please see links below.

Following table shows a relative count for the capacities measured by different file systems.

Various Drive Sizes and their Binary and Decimal Capacities

Drive Size

Approximate Total Bytes

Decimal Capacity
(bytes/1,000,000,000)

Approximate Binary Capacity(bytes/1,073,724,841)

10 GB

10,000,000,000

10 GB

9.31 GB

20 GB

20,000,000,000

20 GB

18.63 GB

30 GB

30,000,000,000

30 GB

27.94 GB

36 GB

36,000,000,000

36 GB

33.53 GB

40 GB

40,000,000,000

40 GB

37.25 GB

60 GB

60,000,000,000

60 GB

55.88 GB

74 GB

74,000,000,000

74 GB

68.91 GB

80 GB

80,000,000,000

80 GB

74.51 GB

100 GB

100,000,000,000

100 GB

93.13 GB

120 GB

120,000,000,000

120 GB

111.76 GB

150 GB

150,000,000,000

150 GB

139.69 GB

160 GB

160,000,000,000

160 GB

149.01 GB

180 GB

180,000,000,000

180 GB

167.64 GB

200 GB

200,000,000,000

200 GB

186.26 GB

250 GB

250,000,000,000

250 GB

232.83 GB

300 GB

300,000,000,000

300 GB

279.40 GB

320 GB

320,000,000,000

320 GB

298.02 GB

400 GB

400,000,000,000

400 GB

372.52 GB

500 GB

500,000,000,000

500 GB

465.65 GB

640 GB

640,000,000,000

640 GB

595.84 GB

750 GB

750,000,000,000

750 GB

698.47 GB

1 TB (1000 GB)

1,000,000,000,000

1 TB (1000 GB)

931.30 GB

1.5 TB (1500 GB)

1,500,000,000,000

1.5 TB (1500 GB)

1396.95 GB

2 TB (2000 GB)

2,000,000,000,000

2 TB (2000 GB)

1862.6 GB

3 TB (3000 GB)

3,000,000,000,000

3 TB (3000 GB)

2793.97 GB

4 TB (4000 GB)

4,000,000,000,000

4 TB (4000 GB)

3725.29 GB

6 TB (6000 GB)

6,000,000,000,000

6 TB (6000 GB)

5587.94 GB

8 TB (8000 GB)

8,000,000,000,000

8 TB (8000 GB)

7450.58 GB

Also see:

"How Mac OS X reports drive capacity".

http://wdc.custhelp.com/cgi-bin/wdc.cfg/php/enduser/std_adp.php?p_faqid=615


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