Ok, this is totally unsolicited but I hope it's helpful. If I were building a storage solution today that I wanted to last me about two to three years this is what I would buy. We use Macs but the box below can be used with a PC as long as you have an open PCIe or PCI-X slot for a SATA host card.

While Network Attached Storage (NAS) -- which connects by ethernet -- and "Storage Robot" solutions like Drobo -- which connects by USB -- are interesting, I still believe that the external SATA (eSATA) solutions are a better fit for photographers because the speed of eSATA blows everything else out of the water in terms of speed. And with the free automated backup software out there for both Mac and PC you can easily set up a nightly save process With minimal tinkering.

All of these items can be purchased from BurlyStorage.com (aka MacGurus.com):

This setup gives you a usable 5.6 terabytes (after formatting) of on-demand storage for $2,557.00
*This set with 750 GB drives is $1,892.56

Storage Box:
Burly 6 Bay SATA Hot Swap Port Multiplier, Two 2 Meter eSATA Cables
Seagate Hard Drives: 6 x Seagate 1TB SATA II HDD $2,373.00
SATA Host Card:
LYCeSATA-4e - Lycom 4 Port PCI Express Host Card $184.00

Paired with something like Carbon Copy Cloner (free) or SuperDuper! ($27.95) for Mac or Microsoft's SyncToy for PC, you can set up smart backups on a schedule with relative ease.

Combine that with some old school DVD burning of your RAW files and you should be set for quite some time.

By the way, here's a Drobo config with four 1 TB drives (~3.75 TB):

Drobo = $500
4 x Seagate 1 TB HDD = $220x4 = $880
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Total = $1380.

If you want the DroboShare network attachment (for NAS), that's another $200.

And it's still not nearly as fast as the SATA solution -- though it's not a bad idea if you don't need lots of your files accessible quickly and are only looking to back up.

Did this help anyone?

Oh... you wanted us to READ that???

I think it's always helpful to see different options. Keep posting as your brain keeps working!

I don't necessarily think having your storage solution directly accessed by your OS is a good idea. Operating systems can be infected by viruses, crash, corrupt files, etc. While that doesn't happen very often if you're paying attention, and it happens even less when you're using OS X from Apple, it still happens. Assuming funds are not an issue, I'd build a hardware based RAID 1+0 with enough space for active sessions, then build a NAS device with a boatload (technical term) of capacity and redundancy and stick it in a different room with a gigabit network connection.