comcom said:
Besides the size difference, what is the difference between 2.5 inch internal hard disk vs 3.5 inch internal hard disk?

Is 2.5 inch hard disk more expensive?

I did not use 2.5 inch hard disk before.
The "2,5inch" drives are common in laptops; but usually slower (but more expensive) that their 3.5 equivalent. You CAN buy adapters to allow a 2.5" drive be mounted in a 3.5" location(usually an external opening) but there's no practical way to allow a 3.5" drive in an location suited for a 2.5" drive.