Magnetic data disks?your typical hard drives?have been going through a golden age, with storage densities doubling for many years of the past decade. But the engineers charged with finding evermore clever ways to stuff extra bits into a given square inch of magnetic platter were beginning to encounter the physical limits of current techniques. New advances in Perpendicular Magnetic Recording (PMR) technology, however, may continue that steep density trend for some time to come.Today, typical hard drives use what is called Longitudinal Magnetic Recording (LMR). With LMR, the magnetic fields that determine whether a bit is on or off lie in the plane of the spinning disk. (Imagine billions of tiny arrows lying on their sides on the disk and pointing either left or right.) PMR shifts the fields so that they point into or away from the disk platter. This reorientation allows for stronger magnetic fields and denser data storage. Until recently, however, PMR technology required a relatively thick deposit of magnetic underlayer that could not be applied efficiently and rapidly with current production equipment. Recent advances by MMC Technology (a subsidiary of disk maker Maxtor), however, may bring PMR within reach of today\u2019s disk manufacturing plants. "We think we can make it on today\u2019s equipment without buying new factories or buying new special machines to do this," says Ken Johnson, vice president of research and development at MMC, noting that in a low-margin business such as disk drives, low manufacturing costs are critical. "It\u2019s starting to look to me like this PMR jump can be made to happen with just good engineering in the next 18 months," he says.MMC is currently touting storage densities of 175GB per platter, 75GB more than the current highest-density commercially available disks, though Johnson says 175GB is only a starting point for PMR. MMC won\u2019t commit to drive delivery dates, but Johnson estimates late 2004 or early 2005 as a realistic time frame for the industry.