I think that's probably the next experiment, I'm not sure if I'd got for a HDD or a very small USB SSD to keep the power usage down - at the moment it's under 0

I think that's probably the next experiment, I'm not sure if I'd got for a HDD or a very small USB SSD to keep the power usage down - at the moment it's under 0.5W, a HDD will be several times that.