Toshiba has designed a mobile phone using the world's smallest disk drive, according to Japanese sources. The 2.1cm drive, also made by Toshiba, is so small that six would fit on a business card.

Japanese mobile phone service provider KDDI said that the W41T handset will go on sale in Japan early next month.

The operator is positioning the phone primarily as a music player, and the launch will tie in with the start of KDDI's new music download service due in April.

The 157g W41T also features Bluetooth wireless connectivity, a 3.2-megapixel camera, a 2.4in 240 x 320 pixel LCD screen, and an FM radio. KDDI has not revealed a price for the product.

Toshiba has not announced any plans to sell the W41T, which works on CDMA networks, outside Japan.

Elsewhere, Imation is selling the 2.1cm Toshiba drive as a standalone product with a USB interface at a street price of around $180. According to Imation, the 3,600 RPM drive has a maximum read speed of 5Mbps.

Nokia's forthcoming N91 phone will also use a 2.1cm 4MB drive, according to a Nikkei Business Press report. The delayed N91 is expected by mid-2006, but Nokia has not yet given the product an official release date.

While the addition of disk drives to phones has raised questions about reliability, smaller drives are generally better able to cope with being dropped than their larger cousins because their moving parts have less mass.

Toshiba claims that its new drive can withstand a force of 1,000G while operating. This is roughly equivalent to an unprotected fall onto a hard surface from a height of one metre.

The manufacturer has said that its storage device division plans to develop a 2.1cm drive with a capacity of 10GB in future by using perpendicular recording technology to pack more data onto the disk.

Currently, the company's 2.1cm disk faces competition from similarly-priced 4GB flash memory products, and from larger 4GB compact flash format hard drives at approximately half the price.