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ZDNet: HGST (formerly Hitachi's disk unit) has announced that it has developed a helium-filled disk drive. The helium units use 23 percent less power than air-filled drives. In addition, they run 4°C (7°F) cooler, which reduces the power necessary for cooling in a data centers and results in a total 45 percent reduction in watts-per-TB. Encasing the drives in helium also allows the company to put two more platters in their disks, increasing the total storage space.
Why helium-filled drives? As a disk drive spins, it causes air turbulence and drag that hinders the performance of the drive. Because helium is lighter than air, it reduces the drag on the spinning disks.
The article also notes, "No, helium-filled drives won't float away if they aren't tied down."