External solutions could be based on either 3.5-inch or 2.5-inch solutions. A 3.5-inch external HDD is a bulky soluČtion and although faster, it requires more space to lug around. More importantly, it also requires an additional power connector (it cannot be fully powered through the USB port alone).
A 2.5-inch HDD is much smaller, cooler and quieter, but the demerits will be the smaller storage space, and of course, the performance. We'd like to add, that while shopping for an external drive, keep speed as the last criteria because irrespective of how slow your HDD is, the slower USB interface will surely bottleneck it. E-SATA is another worthy option to look at, and if your motherboard supports E-SATA, then you could look for an external solution that supports this new and much faster standard. Just bear in mind that E-SATA external solutions will cost a fair bit more.
Unless you need more than 250 GB of space on your exterČnal setup, we recommend a 2.5-inch notebook HDD over the desktop ones for obvious reasons. Besides, you needn't buy an external solution. The option of buying a hard drive and external bay separately could well save you as much as Rs 500. When going external with a notebook HDD, speed doesČn't matter, so a 5400 rpm HDD will do just fine.