When dealing with any hard disk drive, it is always possible that there is physical damage inside. This is even more likely if the drive was dropped. When attempting to read a disk that is damaged, further internal damage can occur. If a head is physically damaged, it can destroy the surface of the platter, which destroys all data on that surface. No software can detect this. It can only be found by examining the drive in a HEPA 100 clean room or laminar flow workbench. NEVER open a drive yourself unless you have a HEPA 100 clean room or laminar flow workbench, and have experience in doing so. If you open a drive outside of those conditions, you will introduce the contamination of microscopic dust particles, which will kill the drive heads and cause damage to the platter surfaces, destroying all hope of recovering the data. So be aware that when dealing with a failing hard drive without knowing what you are doing, you could be causing further damage without even knowing it.