What determines a disk drive's capacity

1. The number of disks and their size

    Disk drives are typically composed of multiple disks. Each disk has an upper and lower surface upon which data may be written to concentric tracks. Subsequently each surface of the disk on which data is written requires a suspension assembly. Disks commonly are either 3.5" or 2.5" in diameter. The 3.5" form factor is the most common in desktop and server applications, 2.5" is common in laptops and mission critical applications.

2. The Areal density per disk

    Areal Density is a measurement of how much data fits into the same amount of physical area on the disk. It is commonly measured in Gbits/in2 and is rapidly approaching 1000 Gbits/in2 (1 Tbits/in2). The formula for determining areal density is Tracks Per Inch (TPI) * Bits Per Inch (BPI). Therefore, areal density is increased by either increasing the number of tracks or bits on the disk.

    In both cases, numerous challenges face the disk drive and suspension assembly designer including how to follow ever narrowing tracks and how to read weaker magnetic signals from each bit (in order to increase the number of bits you typically need to decrease the signal so the individual bit is identifiable from the others). From a suspension assembly designer point of view, increasing areal density requires any one or all of the following: mechanical designs that optimize dynamic performance, suspension micro-actuation to follow finer tracks, and suspensions that can support the mounting of lasers on the head for HAMR.

Some interesting facts about areal density

  • A modern commercially available disk drive can have more than 400,000 tracks per inch and over 2,000,000 bits per inch.

  • The tracks are typically spaced as few as 60 nanometers from the centerline of one track to the next track.

  • The compound annual growth rate for areal density was around 40% per year in the 2000's. It is projected to continue to grow at 30% per year for the next 10 years.