External hard drives are usually not much different then internal ones, though some may have lower rotational speeds to cut costs or power draw (depending on whether you have a portable 2.5" or desktop 3.5" one)
Lost pics from a HDD either lean towards failing drive, though I would imagine more data corruption than just a few photos, or the possibility that your backups are not completing correctly/fully.
How much space are your current photos taking up?
As a rule I say, you need backup storage of at least what you have now, plus capacity to grow and hold at least the following year's worth of growth.
So if you have 500GB now, and plan to take another 500GB of pics this year, you need at least a 1TB drive to fit that in, plus capacity for more, so at the very least 1.5x what you need.
Add on top of that 500GB more every year, and for the next 3 years you now need a 1.5TB drive + 500GB current = 2TB, add your capacity planning, and a 3TB drive for a 3year stretch is what you need!!!
Now here comes the redundancy bit - double up the costs if you want a duplicate of all that data, or for the same price double up capacity and split your work over the 2 drives