Laptop-Review.eu

The basics

3.5"

Advantages:
Low cost per unit of storage
Large capacity
Possibility of high transfer speeds (7200 RPM)

Disadvantages:
Requires an external power source
Large and unwieldy if you want to take it with you

2.5"

Advantages:
Runs without an external power source
Portable
Possibility of high transfer speeds (7200 RPM)

Disadvantages:
Costs a little extra
Lower capacity

1.8"

Advantages:
Very portable

Disadvantages:
Tied to slow drives
Costly
Low capacity

Everything is of course a question of need, because if you do not require hundreds of gigabytes of storage or high transfer speeds, a 1.8” external drive should do just fine. However, if you want to bring large chunks of data with you, you will probably want to look at a 2.5” or 3.5” drive.