I've heard about these devices on various shows on the TWiT network, notably MacBreak Weekly, but I've finally investigated them buying one for myself. I'm now putting all of my stuff on a Drobo.

Drobo (Data Robotics) is a computer-independent RAID-in-a-box that has 4 2.5" SATA drive bays. It supports the use of 4x4TB drives, even though the current maximum is 2TB in a drive. Even so, 4x2TB (after the RAID math) gives you 5.5TB of usable "protected" space. Should a drive fail or the full volume require the smallest drive to be replaced with a larger one, the drive can be hot-swapped with a replacement and the volume will immediately begin to rebuild, with all the data still available. Rebuild time varies with volume size and how much data is actually on it, but it's still pretty impressive to know that

The box even has a small battery to finish write cycles, so a power outage won't kill the device before it has a chance to close a write. Not finishing a write cycle can leave your volume open and irretreivable when you power back on and is the most common failure effect from a power outage. It connects via USB to a single computer and mounts as a normal external hard drive. You set up the Drobo to be a certain maximum size, which is reported in the OS, regardless of how much physical space is currently available from the drives. To manage the device, use the Drobo Dashboard software, which doesn't do much, but it does accurately report usage and rebuild time.

The DroboPro is essentially the same, except it has 8 bays (supporting up to 8x4GB drives) and allowing for single-drive and dual-drive failure (which decreases the usable space). The integrated iSCSI-compatible network card replaces the DroboShare, but makes the overall product reach three times the cost of the Drobo, even with just twice the capacity. It's a much more expensive solution for serious enterprise use, but if it's just as useful as the Drobo, it's probably worth it.

You can pick up an empty Drobo off Amazon for pretty cheap, but for those looking for something in between, Drobo just recently announced two new products, the 5-bay Drobo S and 8-bay dual-ethernet DroboElite. They may seem expensive, especially compared to consumer NAS products, but these are truly all you need.