It was mostly to make it easily available to the arm platform. To add to that Burstcoin is touted as a low power consumption coin but we run it on computers that use more power than necessary. Devices like the PI are cheap and can be put out of the way. We can also use multiple devices so that if there is a crash not everything goes down at once and it will be as fast to fix as swapping out the device.
It's the only arm device I own for now so I had no choice but to build it on that first. I would love to take suggestions for cheap low power devices that have faster interfaces for hard drives.
I haven't checked to see what the bottleneck is but for now I'm using a cheap portable 12.5mm 2TB drive and it reads it in 18 seconds. The PI 2 and 3 do have quad core processors so it could possibly run 4 drives. It reports that the read speed is 25MB per second which is about half of the speed that USB 2.0 can do. As far as I know the PI only runs USB at half the rated speed speed meaning so it looks like that this is the bottleneck.
My PI and drive are powered by a 2.5A phone charger and it's only barely warm to the touch. I thought that the drive would tax it too much.