We just left a Western Digital preview event where the company provided an early glimpse at its yet-to-be-officially-announced My Book Thunderbolt Duo pair of external hard drives. Two models will be available initially, offering either 4 or 6 terabytes of storage capacity. As the "Duo" in the product name implies, each device carries two 3.5-inch disks, meaning that the 4TB model contains 2 x 2TB drives; the larger 6TB version holds 2 x 3TB HDDs.

In case you were wondering just how fast these Thunderbolt-enabled drives are, project manager Julie Wiesen provided us with some figures — though emphasized the numbers as being conservative. An individual My Book Thunderbolt Duo reached speeds between 250 and 280MBps at WD's offices. Where things really pick is when several devices are daisy-chained together. During the press event, the company had four My Book Thunderbolt Duos linked, totaling approximately 24 terabytes of disk space. WD had Blackmagic's Disk Speed Test monitoring throughput in real time, and we saw average write speeds of 450MBps while read results topped out around 760MBps (though Mrs. Wiesen said that her team saw that figure go as high as 900MBps when testing on a newer Mac than what was used for the demonstration).

The My Book Thunderbolt Duo will exclusively support Mac hardware at launch, although WD reps say the company is working with Intel on a firmware update to bring compatibility for PCs once the technology has seen more adoption from Windows users. The folks at Western Digital weren't quite willing to show all their cards: release date and pricing are still unknown, though we expect final details when the company formally unveils these drives at MacWorld later this month.