Hard drive missing space?

Quote from Mad_Hatter : Isn't a TB 1048576mb?

It depends on whose terabyte you're asking about.

Hard drive manufacturers use the SI-friendly definition for the prefixes: a kilobyte is 1000 bytes, a megabyte is 1000 kilobytes, etc. By this definition a terabyte is 1000^4 bytes (1 trillion bytes). This definition is also used in all communications equipment (modems, routers, etc.).

Most software defines the prefixes in a different, arguably incorrect way: a kilobyte is 1024 bytes, a megabyte is 1024 kilobytes, etc. By this definition a terabyte is 1024^4 bytes (about 1.1 trillion bytes). The IEC attempted to define new prefixes to be used with these definiitions (kibibyte, mebibyte, gibibyte, tebibyte), but those never really caught on.

As it happens, 1 terabyte * 1000^4 bytes/terabyte / ( 1024^2 bytes/mebibyte) = 953 674.3 mebibytes = 931.3 gibibytes

This difference actually led to a class-action lawsuit against Seagate, which (hilariously) they lost.

You get a slightly higher number because the drive manufacturers actually throw in some "extra" bytes. Drives are made with some extra space to account for manufacturing defects and to ensure some hidden sectors are available for the drive to call into service as replacements for any damaged sectors that pop up later. And it is not convenient to make the drives with some arbitrary number of bytes (the size of the platter is fixed to a standard size, and aerial density is driven by technology). You can end up getting a little "left over" extra space as a result.

You may find that the reported space once the drive is actually formatted and set up may be a little bit less, for even more complicated reasons.

Bonus trivia: 3.5 inch floppy disks defined a megabyte as being 1000*1024 bytes.