I'd like to second this request - mostly for space utilization, not so much for real time drive activity (I assume that's what's meant by "busy" in the original post) - I'd find it very cool to have drive information in addition to what S.M.A.R.T. provides. I understand that this is not technically a hardware monitoring function though, so I understand the low priority.
I know I can get that out of Explorer, but it would be pretty sweet to have a monitor directly in HWInfo64.
I'm currently running Nick's HWiNFO Monitor gadget on a side screen, as well as a drives meter gadget and a network meter. Mostly for my own gratification, it would just be nice to have a common UI on my always-on monitoring utilities.