In order to understand the process of how data is read and written to a conventional spinning hard disk drive, the information of the following is necessary.
- The hard drives electronics which includes the drives printed circuit board which contains all the necessary electronics to control the spindle motor, armature and more.
- How magneto resistive heads work in conjunction with the drives electronics
- How physical sectors are mapped to logical sectors with the use of a translator
- How Service Area Adaptive Modules work with the drive
- How data is actually read to and written from a drives platter(s) physically
To understand how data is read and written to a platter surface, see Figure 1, Platter Surface 0 which corresponds to Head 0. Heads are typically numbered from the bottom up starting with Head 0. In the example pictured, this drive is a two platter drive with four total heads. Starting at the bottom most surface (Platter Surface 0) and the lowest numbered head (Head 0), data is read from this surface first. Depending on the surface density and other parameters, a predetermined amount of data (sectors) is read from a portion of this surface (track), then the process continues to the next surface from the bottom up. In this case, the next surface would be (Platter Surface 1) using Head 1. This process continues until all platters and heads are used and then the process is reversed from the top down. This process creates a so-called “virtual cylinder” wherein a file of any significant size is contained within the cylinder. The cylinder is essentially tracks and sectors above or below each platter which aligns vertically from the bottom most surface to the top most surface including all platters.
A common issue with data recovery is when several platter surfaces are pristine but there is damage to one of the platters surface. If the file that needs recovery is any appreciable size, all the surfaces are needed to recover all the sectors associated with the particular file since the particular file is made up of all the sectors from multiple surfaces. A good example would be a .MOV or .MPEG file which is typically larger than most document or image files relatively speaking. If all the sectors which make up the file are not recovered, depending on the file type and the amount of the missing sectors, the file will be corrupt and may not open at all.