A number of factors can influence external hard disk performance.
External hard disks are kept in an enclosure that's separate from the main desktop PC or laptop case, and usually connect to a computer via USB. Most external drives, particularly larger ones, require a separate power supply. A number of factors can influence the performance of these devices.
Connection type
The majority of external hard disk drives connect to a PC or laptop using a USB 2.0 connection, capable of a theoretical transfer speed of 480 megabits per second. The newer USB 3.0 standard, which is just coming to market, offers theoretical speeds of 4,800 megabits per second. Both the drive and motherboard need to support USB 3.0 to achieve these higher transfer rates.
Disk speed
Hard disk speed is measured in revolutions per minute, or RPM. A faster RPM means that the disk can read and write data more quickly (because the disk is spinning more frequently). Laptops typically use the slower 5,400RPM minute standard for internal drives (to keep heat and power usage down) but external drives usually use 7,200RPM or 10,000RPM disks. As 5,400RPM drives use less power, they are sometimes used in external drives that don't require a separate power supply.
Disk fragmentation
Over time, as data is written to and from a drive, it can become fragmented. This means that larger files are split up over the disk, making it harder for the operating system and applications to find them. Excessive fragmentation can have a negative effect on performance, but the problem can be minimized using a defragmentation tool such as the one built into Windows.
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