When a hard drive to fail bad sectors begin to develop.
This can happen over time or from other factors like media damage, or even from a light drop.
Special equipment is needed to image the failing drive onto a good working drive reading these bad sectors.
Software cannot perform this task. Only then can software be used to recover the file system and extract the data. If the bad sectors are not read this will result in corrupted data. The recovery must never be done directly from the failing drive since this can damage it even further creating more bad sectors or causing a head crash.
If you are experiencing I/O errors, blue screens of death, or your computer is freezing and crashing this could be a good indicator of bad sectors developing.
This is the most common type of recovery performed.