Two reasons: 1. The "advertised capacity" of a drive is measured in base-10. So 750 GB means: 750,000,000,000 bytes. (Give or take, even then they round.) OS X measures in base-2. So "750 GB" to OS X is 750 * 1024 * 1024 * 1024 = 805,306,368,000 bytes. A large spread. Which also means that what a hard drive manufacturer calls "750 GB", OS X calls 698.49 GB. You're still a little shy of that, though, so... 2. The drive could be partitioned, or have bad sectors, or some other reason it doesn't show a full "698.49 GB". Your best bet if it has data on it is to make a backup, and boot to an OS X Install CD. Run Disk Utility, do a complete re-partition of it and disk check, then restore your backup. Note that while some systems do reserve part of a hard drive for specific purposes (OS X Intel systems reserve a very small partition for the EFI boot loader, and Lion and newer create a "recovery partition" of a couple GB,) OS X doesn't do it for more than a few GB.