The third generation S25 drive from Western Digital has had a third platter added to reach the 900GB capacity point. WD's S25 is a 2.5-inch enterprise disk drive spinning at 10,000rpm with a 6Gbit/s SAS interface. It comes in 150, 300 and 600GB capacity points and competes with Seagate's Savvio 10K – the 10K.4 600GB version of …
Ok assuming an areal density of "509Gb/in" as quoted in the article and an assumption of one side of each platter being utilized for data storage, how is it they reached this capacity per sq/in. By my math, assuming that the center inch roughly of the platter cannot be used as that's where they mount from I get roughly 12.4sq/in that can be used for data on this drive. Even assuming my math is off, on the high side, by 40% there is no way that they are getting that much per sq/in as they appear to only be using 20% ish of the drive for data. Hope this makes sense anyway, correct me if i am wrong though as I am not fully upto speed on the exact dimensions of the platters in these drives.
Unless, of course, the industry definition of areal density is simply the number of bits stored on a platter divided by (pi times radius squared). It doesn't necessarily make sense to exclude the hole in the middle, since its necessary size may be part of a complex design trade-off. Thinks ... a smaller hole means a narrower spindle which will be less rigid and vibrate more, meaning you have to put the tracks further apart to allow for that. So there's definitely an optimum to be sought.
Re: fuzzy math?
that's correct: "enterprise" disks only ever use quite narrow bands of the outer part of the disk, since that gives the lowest latency. these disks are sold on iops, not bandwidth. (which is why, more than ever, they sell to a shrinking niche market. think SSD...)
Well, I hope they're a bit more reliable than the external 640 gig "Elements" drive of theirs I bought a few years back. After about a year, the interface electronics failed, although the drive itself itself was OK at the time, as proved by installing it in a spare external housing. That lasted about another six months before failing. They don't seem to have a particularly good reputation for longevity. Not impressed. Wouldn't buy another!