Why does my 250GB drive show significantly less available disk space, even though it's empty?

This is due to a discrepancy how most computers count bytes (using the binary system) and how drive manufacturers determine the size of their drives (using the decimal system).

Therefore, a drive said to hold 1GB only holds 1,000,000,000 bytes (1000^3), while most computers define 1GB as 1,073,741,824 (1024^3).

In addition, some of the overall storage space is unavailable due to overhead for the file system information.

Please note that with the Apple Mac OS X 10.6 "Snow Leopard", Apple changed how they report sizes from the binary system to the decimal system, so that the reported sizes of the drive manufacturers and Apple Mac OS X 10.6 "Snow Leopard" will now match up.