August, 2010

Why your hard drive cannot read or write

Our customers typically worry most about a disk drive's "click of death", but the fact is that unrecoverable read errors are much more common.

For example, most SATA drives have an unrecoverable read error (URE) or bit error rate (BER) of 1 in 1014, or about 1 block in every 12 TB. These errors are sometimes latent, which means you Don't know about them until you can't read the data. But these errors are often what remains after the very sophisticated servo and signal processing hard drive technology tries to recover the data .

Hard drives are permanently formatted at the factory with vital information in the form of embedded servo tracks that normally tell the read/write head where it is. Like every other bit on a disk however, servo tracks can also be damaged or corrupted, and formatting software or RAID array  simply can't replace them. Even a grain of dust under the head can create a momentary flash of heat - called thermal asperity in engineering lingo - that can actually wipe out a few hundred bits. As a result, vital positioning information is lost - and so is your data. At this point - you need help from a ACE Data Recovery professional.

On top of this, hard drive tracks are not perfectly circular. With disks spinning at 120 revolutions per second - @7200 rpm - it is literally a roller-coaster ride for the heads. The head positioning system knows this and usually adapts. But as disks shake from noise or vibration, or bearings in motors or actuators wear, the heads simply can't adapt as fast or as reliably. They start to lose the ability to lock onto a track long enough to read the data - which translates to losing your data. Hard drive recovery service companies have developed special tools and technologies that can position the heads precisely on a servo track and force the hard drive to read this "lost" data.

Desktop/laptop SATA  drives will retry dozens, maybe more than 100 times before giving up. But RAID drives and costly enterprise drives will quit after a few tries and declare the drive failed, the idea being that mission-critical systems need the best performance. But RAID data redundancy and RAID data recovery is a different subject. So when your drive can't find the data, it really can't find the data. For this reason, data recovery software solutions typically cannot extract your data because they primarily rely on imbedded hard drive features.

Sometimes too, the data isn't readable simply because the write, not the read, has actually failed. A read-after-write check is one solution to this, but it slows drives down too much to be practical in most applications.

In addition, media damage such as scratches, pits, and irregularities can all corrupt data. For example, hard particles scratch, while soft ones like aluminum will smear across the surface, making magnetization more difficult. Since the outer tracks move faster than the inner tracks, the speed difference may also help account for the observed difference in outer track errors. Another possible factor stems from the fact that the disk platter lubricant tends to builds up due to centrifugal force which forces the head away from the media. But by utilizing clean room hard drive recovery technologies, your valued data can be extracted even after this kind of damage.

No doubt about it, today's disk drives are incredible machines. Thirty years ago a 500 MB drive was the size of a washing machine and cost $50,000. Now days you can get a 1 TB drive that fits in your shirt pocket with 2000 times the storage capacity, is 25 times more reliable, and has a data rate that's 75 times faster.

But no matter how advanced the hard drives  become, the more data you put on them, the greater the impact of data loss becomes. Users who rely heavily on hard drives should also understand their limitations so that they can protect themselves and their data.