The basic differences between Portable and non-portable drives are the form factor of the disks they use and their power source. As you have observed, portable drives are smaller and are usually powered directly off the USB port to which they are connected. Non-portable drives are bigger and are usually separately powered.
Portable drives are based on the 2.5" drive used in laptops and other (not surprisingly) portable computers. They tend to have a smaller capacity and usually but not always operate at a slower speed, e.g. 5400 rpm, rather than 7200.
Non-portable drives are based on the 3.5" drives normally used in desktop systems, tend to have larger capacities (up to 3 TB) and operate at the faster desktop speeds, typically 7200 rpm. But this extra weight and speed requires more power than can usually be supplied by a USB port and so they come with an external power source.
The origin of the disks gives a clue as to the difference in portability. Desktops are assumed not to move frequently, at least, not while powered on and so it was also assumed that the disks wouldn't suffer the accidental knocks and drops that would be associated with a portable device. The bigger platters and longer read/write arms do flex more and are not as shock resistant as smaller devices. So a 3.5" non-portable external drive should not be moved while powered on and dropping one is usually fatal.
With the advent of laptops, something more robust was required, that could withstand the shock of a laptop being picked up and typically rotated through 90 degrees to be carried and, unfortunately, knocked or dropped on occasion. While you can carry a powered on laptop around, it's always wisest to close the lid and put it in standby, so the disk(s) have stopped spinning. Enter the 2.5" drive, its smaller physical size makes it inherently less susceptible to shock and the manufacturers have gone to some lengths to enhance this capability. Basically, it is assumed that they will be frequently moved around.
I assume your Xbox is relatively static and so that would suggest a non-portable drive would suffice but where would you get the movies from? If you get them from your camcorder or PC, and that means you need to move the external drive frequently, you might want to consider a portable drive, though within your house, providing it isn't powered on when you move it, a non-portable drive should be OK. You don't say why your WD failed after only a year, that is unusual unless it's been (shall we say) not treated kindly. That might push you to a portable, assuming the Xbox USB ports have enough power to drive it.
One last comment - do remember that ALL disks eventually die - so do remember to keep a backup of the movies, etc., on your external drive. One copy is never enough.
Hope this helps.
I am planning to buy a new external harddrive for everyday use like movies, music,pictures etc. I will be connecting it to my xbox mostly to watch movies. Now i had Western Digital Elements 1TB and it only lasted for a year. There are two types that am confused between the portable ones (which are smaller in size) and regular big ones. If you guys could tell me if there is any other difference than the obvious size difference? I know portable cost just a bit more. But is there any speed difference. Also does big ones last longer than portables? Also which one you guys recommend? I was looking to buy for under $150. Size 500 GB - 1 TB.