Internal drives are much faster and more reliable.
External (USB) drives are much slower and less reliable.
So only use external drives for backup (but NOT your only backup, because their inherent unreliability), for transport of data between different machines.
It's useful to store things that you don't mind at all to loose (because they are so easy to replace) also, but I would do that only if space on your internal drives is reaching zero.
A few weeks ago I posted an inquiry about transforming data from one HD to another.
Yesterday at a super market there was an offer on external HD 500GB, it is going for the equivalent of $75, and I think it is a bargain, correct me if I am wrong i.e if there is cheaper than that, brand Omega.
Going back to the subject, instead of buying a new HD ( an internal one) and go through the trouble of transforming the data just fix the external one and your GB is 500 GBs more, second this external HD can be used on other computers, again correct me if I am wrong.
What my inquiry is, is there a difference between internal and external HDs in performance and reliability ? if so please explain and advise me which should I decide on.