Hitachi has developed a hard drive read/write head that's half the size of the units found in today's top-of-the-line HDDs - a crucial step, it claimed, to delivering a 4TB desktop drive, albeit not until 2009 at the earliest.

Hitachi's new head may be tiny - 30-50nm in size, one two-thousandth of the thickness of a human hair - but the technology it's based on is something of a mouthful: Current Perpendicular-to-the-Plane Giant Magneto-Resistive (CPP-GMR).

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In practice that means the heads are based conduction through metal rather than an insulator, as is the case with today's quantum-tunneling based Tunnel Magneto-Resistive (TMR) hard drive heads. The CPP-GMR head's lower electrical resistance allows it to detect smaller changes in the disc platter's magnetic field, reduced as drive makers ramp up disks' data storage density.

Hitachi claimed TMR technology will prove impractical one disk data densities pass the 500Gb per inch-squared point, let alone the 1Tbpin² density many hard-drive industry operatives are currently chasing.

Today's drives have a density of up to 200Gbpi², so we're a few years away from needing a clear successor to TMR, one reason why Hitachi doesn't expect to get its new technology into commercially available drives until 2009. That's for 50nm heads - 30nm versions will arrive in 2011, the company forecast.

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