Adventures in Hard Drive Recovery

Data Storage and Hard Drive Recovery

To understand hard drive recovery, it is important to understand how data is stored on hard drives.

Data is stored digitally as tiny magnetized regions, called bits, on the disk - a flat, circular piece of metal or plastic with a magnetic coating where information is recorded. A magnetic orientation in one direction on the disk could represent a "1", an orientation in the opposite direction could represent a "0".

This data is arranged on the surface of the platter (disk), in sectors along a number of concentric tracks. Sectors contain a fixed number of bytes - for example, 256 or 512, and are grouped together into clusters. Hard drives may contain more than one platter in a stacked assembly. The larger the capacity a hard drive has, the more platters it will use. Data is written onto each disk surface (top and bottom) by a separate recording head. So a hard drive with three disks will usually have six separate recording heads.

How Data is Written and Read

Every time you save a document on your computer, the data is temporarily stored in an area of your hard drive called the cache. You can think of the cache as a sort of airport terminal, where passengers (data) wait before boarding the plane (disk) and after getting off it. Your hard drive looks for free sectors on which to write the data, and an arm called the actuator, moves the recording heads over these sectors. Because the disk spins, the heads must wait until the selected free sectors on that track pass beneath, before writing the data.

During the writing, a pattern of electrical pulses representing the data passes through a coil in the writing element of the recording head, producing a related pattern of magnetic fields at a gap in the head nearest the disk. These magnetic fields alter the magnetic orientations of bit regions on the disk itself, so the bits now represent the data.

Every time you open up a document on your computer, the same process occurs, only in reverse. After checking the File Allocation Table, a section of a hard drive that contains information about the size and location of files, the actuator moves the head over the track where the chosen data is located. When the correct sectors pass beneath the head, the magnetic fields from the bits cause resistivity changes in the reading elements within the head. These elements are connected to electronic circuits, and the current flowing through those circuits change with the resistivity changes. The current variations are then detected and decoded to reveal the data that had been stored on the disk.