I have a 300GB and a 1TB drive inside my computer and a stand alone 1TB sitting on my desk. To most people this would be more hard drive space than they would ever need. But to a photographer it's just enough to get by for a while until we're forced to shell out for another drive. Then there's the hassle of deciding what to copy on to each drive...
The perfect solution is the Drobo by Data Robbotics. Drobo is a single unit containing up to four hard drives, but the computer sees it as one single drive. The most amazing thing is that if a drive fails or even if you simply pull one out, the Drobo doesn't skip a beat, it just heals itself and use's it's own back-ups to rebuild a totally safe place for your digital files to live. You can mix and match the size and make of each drive and you don't have to use expensive drives either. Drobo sells for around £340 / $398 on Amazon for the four drive version (drives not included). There is also an eight drive version.