Seagate has samples of its 4 platter terabyte drives, but it is still not ready to release them. The hard drive manufacturer still thinks it's not the right time to do that, as they practically dominate the super storage market with their 750 GB drives.
Seagate wants to be sure that the products they ship are top notch. Hitachi's drives use five platters and are more expensive to build than Seagate's 1TB version.
Storage is getting really cheap and I can still remember Windows 95 and the 600 MB huge hard drive I installed it on. Now it sounds ridiculous.