WD unveiled world's first 3TB internal drive, a Caviar Green model with four platters. This means that WD has been able to reliably manufacture 750GB platters, spinning at 5400rpm, and with an areal density in 500-550Gbit/in2 area. Its 2TB RE4-GP having around 400GBit/in2 and 500GB platters.
Seagate's 3TB drive spins at a faster 7200rpm but it's a fourth generation PMR (Perpendicular Magnetic Recording ) technology drive with 600GB per platter. The Caviar Green uses fifth generation PMR.
3TB Caviar Green also comes in a 2.5TB version and both are Advanced Format drives that face problems being used by 32-bit versions of Windows and older PCs.
Windows XP has problems in this area and isn't being supported by these Caviar Green drives. It may support the drives' use as USB-connected external storage if a correct USB bridge is used.
The 32-bit versions of Vista and Windows 7 support secondary storage use only. The 64-bit versions support booting from the drive when the host motherboard has the UEFI firmware.