Most external hard drives use 3,5" disks, and the reason is easy to see: they offer the largest capacity for the lowest price. However there are also models with 2,5" disks on the market and these have known pros compared to their larger brothers. First of all, they obviously are smaller, making it easier to keep the disk with you, especially when using a laptop. Secondly, the 2,5" disks require less power, so they produce less heat and don't need an external power supply.