Assuming you don't need RAID on the backup device itself, then a cheap desktop PC (usually from a custom white box builder - most OEM PCs don't come with enough SATA connectors/hard drive bays) with 5 or 6 4TB SATA hard drives does the trick. Sure, it'll cost you a fair amount for the hardware (in the UK, probably around 1,000 pounds or so), but it might be the most flexible solution (e.g. could be located offsite if you're paranoid, though you'd need a fast connection to it - at least 100 Mbits/sec I'd have thought - for that amount of data).
Of course, if you then want to keep multiple archive copies, then you'd have to look at compressing the backups and/or perhaps using backup software that does incrementals (e.g. Amanda on Linux or whatever). Another much pricier alternative is multiple spanning Ultrium 5 tapes in 24-slot autoloader attached to a machine with little local storage (1-2 TB free for holding space), but we're talking 5,000 pounds or so for this solution.