Sand is an excellent acoustic material for three reasons. It is heavy material, so it absorbs sound very well. It doesn't resonate at all. And it is cheap and widely available. However sand shouldn't be used in isolating hard disks without any cooling since heat conductivity of sand is only around 0.27 W/mK. Not much compared to aluminum's 237 W/mK.
Here's some pictures of my current system. A hard disk is surrounded by aluminum plates and 20 - 30 kg of sand in plastic bags:
The result looks like this, and noise level is unaudible.
HD model is 80 GB Maxtor D540X. Maximum temperature during the summer under several days of heavy stress is 37 C.