When I read about the Western Digital 10GB HDD I thought "huh? Helium???". And then i searched and found Ultrastar He6 which claims to be the first He-filled HDD... And i was again like "huh? Helium?". And then this guy comes to solve the mystery.
Filling an HDD Drive with a light gas (like Helium) lets the disks spin with less resistance and therefore achieves the same speed with less power, ultimately releasing less heat. Here is a very nice experiment: