In 2011, you may go into a computer store and be surprised to see external hard drives that have more capacity than their internal counterparts. For example, you may see an external drive with a 4+ TB capacity – but fail to find one with a similar capacity that can be installed into a desktop computer.
The reason for this is relatively simple: desktop (and notebook) hard drives have to adhere to standard size specifications. External hard drives, being manufactured specifically for use in external drive enclosures, don’t necessarily have to conform to such tight constraints. In short – a hard disk drive designed for use in an external enclosure can be physically larger (or smaller) than its desktop counterpart.
What this means is that an external drive can hold more platters than a desktop drive. If, for example, a standard desktop drive supports 3 platters, creating a drive with 4 platters would enable 33% more data storage on the drive with the extra platter. Say, for the sake of argument, that each platter is capable of holding 1 TB of data – clearly a drive with 4 platters should be able to hold 4 TB of data. This 4 TB drive, with its extra platter, would take up more vertical space than its 3 TB counterpart – and would require a larger enclosure to hold the platters.
In the past, drive makers were able to enlarge capacity by increasing the data that can be held in the same physical space. However, they’re now running out of tricks . The term defining the amount of data that can be written and successfully read from the media is ‘areal density,’ and it defines the amount of data that can be stored on a specific area on a platter. Increasing areal density has become more and more difficult.
For the past 26 or more years, the PC hard disk drive has been the beneficiary of Moore’s Law, which predicted that every 18 months, the complexity and capacity of computing devices would double. In fact, for hard disk drives, drive manufacturers have actually exceeded Moore’s Law by a healthy margin. It wasn’t uncommon, until recently, to expect capacities to double in less than a year. (Since 1984, when 10 MB was the maximum size for the PC hard drive, until now – with 3 TB or larger drives available, the steady increase has exceeded Moore’s Law).
Hard drive makers ran into many challenges in the quest for continued capacity increases. It appears that, with all their efforts, they may be approaching an areal density limit that may be extremely challenging (if even possible) to overcome. For now, drive makers seem to be flirting with the limit and spending a considerable amount of effort figuring out exactly how to squeeze the extra few percent of capacity into standard drive formats.
This explains why, today at least, drive makers are beginning to create drives in non-standard sizes. As noted, these drives hold extra platters. The extra discs are one method of providing the additional capacity. We will probably be seeing external drives with higher capacities than drives designed for use in desktop or notebook computers.