Hard Disk Space

Try WinDirStat: http://windirstat.info/
...but know that HDDs are slightly upsold by their manufacturers. Hard drive makers and the Windows OS use slightly different definitions of gigabytes. From the Wikipedia "Gigabyte" page: "Since the early 2000s most consumer hard drive capacities are grouped in certain size classes measured in gigabytes. The exact capacity of a given drive is usually some number above or below the class designation. Although most manufacturers of hard disk drives and flash-memory disk devices define 1 gigabyte as 1000000000bytes, software like Microsoft Windows reports size in gigabytes by dividing the total capacity in bytes by 1073741824, while still reporting the result with the symbol "GB". This practice is a cause of confusion, as a hard disk with a manufacturer-rated capacity of 400 gigabytes might be reported by the operating system as only "372 GB", for instance."

They advertise hard drives as 1 GB = one billion bytes. Therefore the hard disk is 640 billion bytes which translates to 610 GB for techies.

Both are right actually since a hardware GB means Gigabyte, while an OS GB means Gibibyte. HDD makers use Gigabyte since it looks cleaner and it makes their numbers bigger, while software tends to work in base 2 and use Gibibytes.

The way I put it:
1 human GB = 1X10^12 bytes or 1,000,000,000,000 bytes

1 computer GB = 1X2^30 or 1,073,741,824 bytes.

Manufacturers use human gigabytes because that makes their hard drives look 7% larger.

Not false, but misleading.