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 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.
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. ®