Why the $$ go up so fast in business-critical storage
Search for "Bluearc" for a quick idea of what these things look like. Those guys publish photos and Google Image usually returns something quickly.
As mentioned 1TB drives are cheap per gigabyte but not necessarily usable beyond a certain point. You can build cheap small office systems but even a small office will feel the effects of using 7200rpm drives in RAID.
The technology to look at is SAS, SCSI, Fiber Channel equipment etc. which is much more expensive. Other points to consider are (1) for each drive whose capacity is used for storage, at least one other drive is mirroring it. Probably more than one.
(2) For each set of mirrored drives there is likely a drive just sitting there empty, ready to power on and begin rebuilding if another drive fails.
(3) Some larger businesses will have mirrored duplicates of the entire storage system, so that if the controller board goes down they can still operate.
(4) It is not always possible to purchase the newest drives with the best dollar-per-gig ratio. When maintaining a huge system like that, the drive capacities have to match or the extra space just gets wasted. If you have a three year old array and some drives are starting to fail, you can't just buy 300GB SAS drives. You probably have to match the 76GB SCSI drives.
(5) The cost of the chassis and the controller hardware is not small. Just the metal rack that holds the thing costs hundreds. Purchased parts-wise the hot-swap bays are $100 to $200 per every 3 to 5 drives. A system board capable of connecting 100+ hard drives and managing them as a single volume is very specialized and expensive hardware too.
... Not that I didn't take full advantage of the cost of 1TB drives when my boss needed 4TB's of safe storage, real cheap, right now, a couple months back! (But, sadly, even with five people hitting it, the performance is not nearly what I'd expect from SCSI.)
