White Paper: When One Billion Does Not Equal One Billion, Or: Why Your Computer's Disk Drive Capacity Doesn't Appear To Match The Stated Capacity

By James Wiebe, CEO, WiebeTech LLC

Computer manufacturers were recently sued over the apparent misadvertisement of hard drive capacities. The purpose of this white paper is to examine the mathematics behind the hard drive capacity claims and understand the information which modern operating systems report on drive and file capacities.

These issues occur whenever the size of a hard drive is advertised at a particular size yet shows up in the operating system with a lower capacity. (For example, a drive advertised with a capacity of 120GB shows up with a capacity of 115GB in the operating system.)

Why does this apparent mis-statement occur? The answer lies within an analysis of the mathematics of the drive capacity, as expressed first by the drive manufacturer, and then by the operating system vendor.

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