Seagate recently took the wraps off of a new 4TB hard drive. The new drive uses the company's 1TB platters, and the ST4000DM000 uses four 1TB platters. Other characteristics include a 7,200 RPM spindle speed, 64MB cache, and support for the SATA III 6 Gbps interface.
According to the company, the 4TB drive boasts an average read/write data rate of 146MB/s (which is good for a mechanical hard drive), max sustained transfer speed of 180MB/s, and sub-8.5ms and 9.5ms average seek times for read and write operations respectively.
The drive is compatible with Seagate's DiskWizard technology, allowing the full 4TB to be used on legacy operating systems. At 4TB, this drive is perfect for digital pack rats and media enthusiasts.
The 4TB Seagate drive can be found for around $190 USD online for the bare-bones drive, or approximately $205 for retail packaging. You can find more information on the 4TB mechanical hard drive on this Seagate data sheet (PDF) or the drive's product page.
I have to admit that I'm tempted by this, despite not having filled my 2TB drive yet.