You may want to know what data recovery is all about before trying to get deleted files back.
If a storage medium damages or it appears personnel wrongly operations, or OS itself gets trouble, any of these will make the computer data unable to be seen, read, and even lost. People use special methods to restore data, invisible in normal circumstances, to be read and visible again.
Different kinds of hard disks, storage tape library, flash driver, mobile HDD, digital SD card, etc...
The basic unit of hard disk data storage is "sector" which can be defined as a page of a book. When using the computer for the first time, our top priority is partitioning for better management of the computer which is like to make catalog for a book. Meanwhile, the partitioning tool will label hard disk witheach partition size, starting location, info like this, we typically call it MBR. When the MBR records are destroyed, some or all partitions would naturally disappear. But we can via analyzing data character, recalculating, manually labeling to rescue data.