Capactiy of external hardrive

That's normal. Storage manufacturers consider 1 GB at 1 billion bytes (1,000,000,000 bytes).

Computers, however, use base-2 arithmetic, and one kilobyte is 1024 bytes, one megabyte is 1024 kilobytes, and one gigabyte is 1024 megabytes. This all multiples to 1 gigabyte according to the computer is 1024^3 or 1,073,741,824 bytes.

Take 320,000,000,000 and divide it by 1,073,741,824 and you get 298.023223876953125, so your computer says your "320GB" drive is 298GB.

There's a standard for base-2 (binary) measurement terminology (gibibytes, mebibytes, kibibytes, etc), but few people seem to use it.
http://en.wikipedia.org/wiki/Orders_of_magnitude_(data)