Why all the hate, he asked a reasonable question, and a good technical one at that.
The answer is that the highest capacity external HDD is theoretically infinite
.
Try looking for a NAS device that can hold between 2 and 5 drives, it may be able to connect as a mass storage device over USB, or it may use Gigabit ethernet, which would be fine.
Western Digital has a couple stand-alone USB enclosures that hold 2 drives.
If on the other hand you need a single drive only I don't know what to say, single drives will be more expensive.
(If I was going to save movies I would hold the data in the original form, and that is because I would use up-conversion on a 1080p projector at about a 200" 16:9 screen size. If you aren't familiar with that quality then don't bother arguing the point.)
I think you are seriously treading into "mini home server" territory, or big NAS territory.
I will look for some for you and get back (It might be a while, I am out on the beach at san francisco, and the sun might burn off the fog soon
)
There is a dual drive 2TB for $500, and a $1000 4TB in a network Storage (NAS)
http://www.wdc.com/en/products/products.asp?driveid=340
http://www.wdc.com/en/products/products.asp?driveid=501
If it was me I would make my own with FreeNAS and put it in a small box with a Intel Little Valley ITX board (D201GLY2 is Core2 processor for $65-75 !!), use a standard ATX PSU at ~400watts to get 4 or so hard drives spinning.
To save more money use an old PC as this sort of server.
I am afraid that this won't be much help if you want to backpack your collection, but an ITX board is pretty small and with a slimline 3 drive case it might be possible.