As you're probably aware, there are several technical limitations that prevent most systems from being able to use a 3TB drive, but Seagate's DiscWizard software offers workaround. To use a 3TB HDD for storage you need an OS that supports GPT partitions (read: not Windows XP), while a 64-bit OS and UEFI motherboard are required to create boot partitions larger than 2.19TB.
DiscWizard installs a driver to achieve compatibility with non-UEFI systems (probably what you currently have). Additionally, the software creates a bootable 2.1TB partition alongside a second 900GB partition, but you still can't have a single 3TB volume. DiscWizard doesn't seem to do anything particularly magical, but it should make it easier for users to get the drive up and running.
"We believe UEFI will remain in a nascent category for a bit longer before there is broad adoption," said David Burks, marketing director for Seagate. "We felt strongly that we needed to create a product that didn't require UEFI for customers who wanted to use the full capacity of these big drives." The Seagate Barracuda XT is shipping now with an MSRP of $279 and a five-year warranty.