Unexpectedly high demand for its Raptor Serial-ATA hard drive has pushed Western Digital into doubling its quarterly production plans.

"We sold a lot - we have been very pleased how it came out," says Ian Keene, the disk maker's SATA business development manager for Europe.

Cynics might suggest that, at just 36GB, Raptor was never intended to do more than grab a few headlines as WD's first entry into the SATA market, hence the surprise when it began selling in quantity. Keene refuses to give actual numbers though, preferring to say merely that the original production plan was "conservative".

He says that Raptor has been selling against SCSI into the entry level server market, where 36GB is still an important capacity point and adds that a dual-platter 72GB model is due out around October.

WD recently added a 250GB SATA drive to its Caviar family too, but Keene points out that this uses a 7200 RPM desktop IDE mechanism designed for a 30 percent duty cycle, whereas Raptor is a 10,000 RPM drive intended for 24x7 server usage.

"A single user won't see much improvement from SATA - the bottleneck is the mechanism, not the interface," he says. "Where people are using SATA is NAS - if you put that drive into a Terabyte box you will see an improvement."