2.5″ hard drives are otherwise known as laptop hard drives, but you’ll see them sold separately the most as external hard drives. These external cases usually work with laptop drives because the currency USB can give is limited to 1A and since the voltage is 5V, it is easy to calculate that the maximum power a USB device can draw is 5 watts.

2.5″ hard drives are just like their desktop relatives as far as working principles go. They have similar metal disks covered by a magnetically active surface which the head uses to store information. These disks spin at 4200, 5400 or 7200 times every minute, and even though they do that in a semi-vacuum it still takes electricity to keep the disks spinning at the exact same speed.

Hard disks are connected to the motherboard directly via either a ribbon cable (PATA) or a SATA serial thin cable. The latter uses 7 wires instead of 80 therefore helps laptop manufacturers to save space, and desktop manufacturers to obtain better airflow in their PC cases. It also saves money to manufacture a cable with only 7 copper wires instead of 80.

2.5 inch hard disks differ in one very important way though, the disks they use are only 2.5 inches in diameter instead of 3.5. That computes to about 51% of the original surface left if we’re not counting the motor in the middle, and taking that into consideration makes the case even worse. Hard disk manufacturers had to think about it as well when they made their laptop drives, since they still had the pressure to make similar capacity hard disks to 3.5″.

They made data more dense on the disks so that each bit or block takes less physical space that it used to. It made reading and writing the disks to be harder and called for more precise motors and head mechanisms. Partly because that, and partly because of power consumption the 7200 rounds per minute speed had to be lowered 4200 to ensure safe data write and read characteristics even with the more crowded disks.

External hard disk drive manufacturers took advantage of the lower power drain by consequence and started selling their products with a 2.5″ HDD instead of a 3.5″ one.

First external hard drives using this technology were relatively expensive, but now it is easy and cost effective to buy one. A 320Gb external USB 2.0 HDD costs about $65 and 500Gb versions are sold for $95.