Hitachi has unveiled the highest capacity drive yet developed for portable PCs, after making a 500GB hard-disk drive for laptop computers.

The Travelstar 5K500 is due to begin shipping in February and should go some way towards quenching user demand for storage space.

Hitachi has managed to increase the capacity of its 2.5-inch drive by adding a third disc platter and by cramming data together more closely on those platters.

The extra platter means the drive is physically larger than previous models: 12.5 millimetres thick versus 9.5mm for most current 2.5-inch drives. That means it won't fit in the drive space in many laptop computers, but Hitachi is confident that PC makers will redesign their machines to accommodate the drive because storage is a key selling point right now.

"The hunger from end-users for more capacity has been phenomenal," said Larry Swezey, director of marketing and strategy at Hitachi Global Storage Technologies, Hitachi's hard-disk drive subsidiary. He said the new drive is targeted at the type of consumer laptops that typically comes with a large screen and high-end audio and graphics.

One PC maker has already fallen for the drive. Taiwan's Asus has announced its new M50 and M70 laptops will use the drive. The latter machine will contain two of the drives to offer 1TB of storage space.

With an eye on this target market, Hitachi has made some modifications to the drive to improve performance, Swezey said. There are new vibration sensors that help the drive's read/write head keep on track. These are required because high-end entertainment laptops have increasingly sophisticated sound systems and larger speakers than can cause vibrations in the drive when music is being played loud. These vibrations can lead to read/write errors and a drop in performance.

The 5,400-rpm drive offers a 5.5-millisecond average latency and 12-millisecond average read time. It comes with a SATA (Serial ATA) interface that supports 3Gbit/s data transfer although this drops to 1.5Gbit/s when the optical disc encryption is turned on. The drive consumes 0.95 watts at idle and 0.7 watts at low-power idle, which is only slightly more than Hitachi's previous two platter laptop drives, Swezey said.

In addition to the 500GB drive, Hitachi will also offer a 400GB drive and "enhanced availability" versions of both drives designed for 24/7 use in products such as digital video recorders. The basic versions will be available in February and the enhanced versions about a month later. The basic 500GB model will cost about $399 and the 400GB drive will have a $349 price tag, Hitachi said.