Hard disk manufacturers list sizes in GB (base10). Computers report sizes in GiB (base2). (More)

640 GB * 1000 is 640000 MB.
640000 MB * 1000 is 640000000 KB.
640000000 KB * 1000 is 640000000000 B.

640000000000 B / 1024 is 625000000 KiB.
625000000 KiB / 1024 is 610351.5625 MiB.
610351.5625 MiB / 1024 is 596.046447753906 GiB.

640 GB is 596.046447753906 GiB.