External “Toaster”-type adapters
These are square blocks with a slot on top that accepts a SATA hard drive, and connects to your computer through USB or e-SATA.
The pluses are convenience, cost, and speed.
The minuses are the risk of metals shorting out the drive electronics, and a lack of heat dissipation.
External case with fan
These are the best cases – until the fan fails. Then, it’s not so great. My personal experience was that the fan failed after a year.
The pluses are the fan.
The minuses are the risk of the fan failing – potentially leading to a hot hard drive.
External case without a fan
These are the second best cases. The ads say that the case is designed to pull heat away from the hard drive. It works as advertised, but, the heat must the be removed from the case. So the entire case needs sufficient ventilation.
Pros: nothing to break.
Cons: you still need to figure out a way to remove heat.
Power supplies: a universal problem
Generally, for whatever reason, the power adapters I’ve used with these external hard drives have generally been junk. They’ll last 1 to 2 years, and fail.
There’s no simple solution out there, except to buy another adapter. Make sure the adapter is on a hard surface with good circulation.
(A possible solution is to hack a PC power supply and use that to power all your electronics.)