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Laptop computer storage is racing fast towards the 500G-byte level with Fujitsu becoming the third hard-disk drive maker to announce a drive at that capacity.

Fujitsu is accomplishing this capacity by combining three disk platters -- the magnetically-coated disks on which data is stored -- each with a 170G-byte capacity inside the drive. Hitachi, the first company to announce a 500G-byte drive, and Samsung Electronics are also using three platter designs.

The third platter increases the thickness of the drive to 12.5 millimeters versus most other laptop drives, which have just two platters and are 9.5 millimeters thick.

As a result the Fujitsu drive won't fit into the drive bay on many laptop computers, so its availability doesn't mean an instant capacity upgrade will be possible for all laptop users. Hitachi's drive is also 12.5 millimeters thick, but Samsung said it has been able to keep the drive height at 9.5 millimeters on its Spinpoint M6 drive.

Fujitsu will begin selling its 2.5-inch drive in May. It will have a Serial ATA (SATA) interface and can transfer data at up to 300M bytes per second. The average seek time for reading data on the 4,200 rpm drive is 12 milliseconds and 14 milliseconds for writing data. Fujitsu didn't announce pricing.

Hitachi said in January that it would ship its drive this month and Samsung announced a March shipping date. Both the Hitachi and Samsung drives spin the disk faster, at 5,400 rpm.

All three drives will also be aimed at the fast growing digital video recorder (DVR) market. Many of the DVRs use laptop drives because they are smaller and lighter than the 3.5-inch models typically made for desktop computers and servers.