Original URL: http://www.theregister.co.uk/2012/06/18/buyers_guide_hard_disk_drives/

Buy Smarter: what you need to know about... HDDs

Bulk storage for your computer

By Simon Williams

Posted in Hardware, 18th June 2012 09:00 GMT

Buyer's Guide Despite advances in other kinds of storage, hard drives are the backbone of the vast majority of desktops and notebooks.

And they are still the most cost-effective way of adding extra storage capacity to an older machine. They are cheap to buy, easy to use and remarkably robust in service.

Typical warranties run from three to five years and, if that’s running continuously, as it could easily be for a drive in a server, that equates to more than 40,000 hours of use.

Compare that service life with that of a typical projector lamp, at around 2,000 hours, or of an electric drill, rumoured to be less than 20 hours.

While the prices of hard drives continue to fall, their capacity goes on climbing, so the overall cost per gigabyte is getting lower and lower.

A 2TB drive, for example, can be had for about £70, a cost of 3.5p per gigabyte. Compare that with a rewritable DVD at around 9p per gigabyte, or a Blu-ray disc at about 29p. Memory sticks and cards can only manage 38p or so, at best.

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Drive types

There are two drive form-factors in common use: 3.5in, normally reserved for desktops and servers, and 2.5in for notebooks. You can also buy 1.8in drives, which are mostly for specialist applications such as some media players.

Most hard drives now use Sata to connect to the host computer, although IDE devices are available, mainly for older machines.

The advantage of Sata devices is a simple data connection: a small, seven-core cable rather than a 40-wire ribbon.

Notebook IDE drives use a 44-wire ribbon, with the extra connectors providing 5V power. Before buying an internal drive, don’t forget to determine which type of interface is used by the device you are buying the drive for.

External hard drives may be 3.5in or 2.5in, but they are housed in robust cases and generally connect by USB, or less commonly Firewire or eSata (external Sata). They can be Sata or IDE inside the case, but this will be transparent in use.

The 3.5in external drives need separate power, either from a supply inside the drive case or from a separate block, normally supplied with the drive.

External 2.5-in drives can often draw all the power they need from the same USB connection they use for data, and are therefore more convenient. That, plus their smaller size, has made them the standard for portable drives.

Drive Capacities

Disk capacity increases unabated, with 3.5in drives of over 2TB now readily available. As with memory cards, the best advice is to buy the highest capacity you can afford. It is always useful to have spare storage space in just about any device.

If you are buying for a desktop PC, though, it is worth considering buying two drives of half the total capacity you need, as it will provide much more versatile storage.

Distributing files between two drives gives added security in the event of disk failure, too. Even if you don't want the extra expense of a Raid 1 array, which gives you automatic backup by copy data to both drives, having files on two drives means you lose only half your data if a drive goes down.

Going for the two-drive approach does mean having two spare controllers on the motherboard of your PC, of course.

There's a price sweetspot for each type of hard drive, largely dependent on popularity. At the moment, it probably sits around the 1TB mark for 3.5in drives and 500GB for 2.5in.

This would be where you get the best deals in cost per gigabyte terms, with rates going up for noticeably higher or lower capacities.

Drive Speeds

Drive speed ratings depend on two main criteria: the spin speed of the disk platters and the amount of on-drive cache.

Typical speeds for 3.5in drives are 5,400rpm, 7,200rpm, 10,000rpm and, at a price, 15,000rpm. There are 2.5in drives available in the first three of these.

Average latency, the time taken for the drive heads to reach any point on the disk, varies from 5.5ms on a 5,400rpm drive down to 2ms on a 15,000 rpm one, so you do see a roughly linear increase in access speed with rotational speed of the disk.

The size of the memory cache fitted to a drive has less effect on its apparent speed, particularly when saving smaller files which fit completely inside the cache.

Typical cache sizes range between 8MB and 64MB, although there's little noticeable improvement in real-world terms with a cache much bigger than 8MB. Cache memory is cheap to fit so cache size tends to keep pace with increases in drive capacity.

Fitting a higher-speed hard drive will improve data-intensive applications but is not the first option in any PC upgrade. You will generally see a bigger performance improvement by adding more memory or fitting a faster processor.

And finally…

If you choose to fit an external drive with a USB 2.0 connection, bear in mind that the link will govern the speed of the drive.

A USB 2.0 port has a maximum theoretical bandwidth of 480Mbps, while a typical 7,200rpm hard drive can manage around 1Gbps.

Both give somewhat less than these rates in practice, but you can still see that you will get roughly half the speed out of a USB-connected drive compared with one linked directly to a drive controller.

USB 3.0 – aka SuperSpeed USB – Firewire or eSata, are better choices if you want to get the same speed from an external drive as you would from internal one. ®