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

750 GB * 1000 is 750000 MB.
750000 MB * 1000 is 750000000 KB.
750000000 KB * 1000 is 750000000000 B.

750000000000 B / 1024 is 732421875 KiB.
732421875 KiB / 1024 is 715255.737304688 MiB.
715255.737304688 MiB / 1024 is 698.491930961609 GiB.

750 GB is 698.491930961609 GiB.