Currently $449 from B&H Photo, not portable, but they are hard drives. HDDs are definitely capable of storing and streaming full high def video for editing in a portable drive - the WD My Passport Pro mentioned above can push through 230 MB/s which can handle a few ProRes 422 streams at 122 Mbps). Just not those cheap 5400 rpm standard laptop drives that every man and his dog seem to be packaging up these days, as you pointed out.
At the time of my own purchase, I had to decide between it and the Western Digital My Passport Pro (2TB for $299 or 4TB for $429), which is portable (which isn't that important to me but it was double the capacity for a bit less $$).
In the end, the higher transfer rate of the Velociraptors (~400MB/s) won out for me. If I couldn't get half a dozen ProRes streams out of that there's something wrong.
SSD was considered, but even one I made myself would be 4x or more the cost of the Velociraptor or My Passport Pro (priced approximately from One World Computing's website). It's interesting to note the incompressible data transfer rates (and that's what applies to video) are quite a deal lower than the headline advertised rate. The 'consumer' 6G SSDs are just above 200 MB/s, while the more expensive pro SSDs are much better, but also more expensive.
I agree the OP would do well to consider an SSD in his calculations. Price, capacity, speed, portability and longevity are all things to consider.
It's a good time for tech. We have so many choices.