In most computers, when you save files they get stored on a hard drive. Although you wouldn’t know it, the drive does not store your files in a straightforward way. The data is written magnetically by a fixed comb of heads stacked above one another. These heads pass between several magnetic discs, writing data as they go. In most cases, instead of storing files on one whole disk they are split up and spread across the disks. This means that when we carry out data recovery we usually need all of the disc surfaces in good condition to get the data back.
When required we can use a process to take the data from the drive by one disc surface at a time. This can allow us to avoid a failing head until we have the rest of the data extracted. When we have extracted all of the data the parts are combined to allow access the files. In some cases this is the only way to get the data back.
Hard drives do not allow access to individual disks during normal operations so we need to use specialist hardware and software.