I've started generating a considerable amount of photography data myself. My main Lightroom library is well over 300 gigs, and there are also all of the backups. I tried out a couple Drobo RAID devices, which connect via USB (or possibly network, with an extra device.) The Drobo, while a simple to use device, is excessively slow.
After some research, I came across the NetGear ReadyNAS devices. These are small Network Attached Storage (NAS) devices that support 1-4 hard drives in RAID configuration. You can start out with a single drive, and expand as you need to. Once reaching the maximum drive capacity, you can swap out one of the drives for a larger drive, and the system will expand into the additional space.
NetGear offers a couple generations of their NAS devices now. The older generation, the ReadyNAS NV+, offers X-RAID 1, which requires being taken offline when a new drive is added or upgraded. The newer generation, the ReadyNAS NVX, offers X-Raid 2, which allows your NAS to remain online and usable while the drive expands into new space. I chose an NVX myself.
The total cost of a ReadyNAS ranges from $300-700 for the enclosure, plus the cost of drives. Buying a ReadyNAS with drives tends to be more expensive, as high speed drives can be purchased much cheaper individually. I recommend using Seagate Barracuda or Hitachi drives myself, as they have a larger cache (32mb for the 2TB) and higher density platters. The larger cache and higher density generally equate to higher speeds (a Western Digital Green or Blue runs about 20-30mb/s sustained, while a Seagate or Hitachi run about 90-130mb/s sustained.) My ReadyNAS NVX setup uses three Hitachi 2Tb 32mb Cache drives, and achieves a sustained 80-85mb/s read speed, and about 60mb/s sustained write speed.
Despite the cost, the investment is pretty sound. For about $400-$800, you can get a rock-solid, quiet, simple device that can store a tremendous amount of data, and can grow with you for many, many years. If you start out with 1TB, you can easily expand to 8Tb or more in the future. The performance is phenomenal, the device is flexible, and it should last.