There are two types of failures. First one is the predictable and the other is unpredictable. Data disc recovery depends on which type of failure that is occuring.
Disk failure can occur during normal operations, or due to external factor such as exposure to fire or water or high magnetic waves, or suffering a sharp impact.
Severity of disk failures varies. Most severy disk failure is the head crash, where the internal read-and-write head touches platter or the magnetic data storage surface. A head crash usually can cause severe data losses. Attempst to recover data without a specialist with proper equipment can further damage data.
Also, a hard disk controller electronics can fail occasionally. In this insteance it is possible to recover this data. Hard drive platters or magnetic data storage surfacess are coated with an extremely thin layer of non-electrostatic lubricant where the read-and-write head hits the surface of the magnetics surface during a collision. Generally, disk head hovers few nanometers from the magnetic surface.
Faulty air filter can cause disk failures. Harddrive enclosure and its outside environment atmospheric pressure is equalized by hard drive the air filter. If the air filter fails to capture dust particles, dust particles can land on the magnetic surface and cause fault data when the head goes over it. This way each dust particle that lands on magnetic surface can cause bad sectors. Many sector failure can lead to entire drive failures.