External hard disk VS flash drive. Given both have the same memory capacity like 2TB, is it preferable to buy the flask drive as it's much way cheaper?

Erm by flash drive, you mean USB thumbdrive? Most likely is too good to be true.

Really depends on your purpose.

My friend stores his steam games on his external SDD. The read and write speeds are really fast so there's not much lag. A flash drive on the other hand, would not be able to support gaming.

If you're using it just to store data such as photos or small documents, a flash drive would suffice.

Hence you need to check how much read and write speed you need.