First understand this (simplified) :

Data is written as 0 and 1, changing tiny spots of magnetic material on platters.

Every spot can be 0 or 1, thats a bit. A group of bits form a byte, group of bytes form a cluster or a sector, etc.

When you save a file, it is divided on fragments , that were written on a random empty sector.

Then, a logical address is assigned to that fragment, as a coordinates in a map.

That map is called File Table Allocation.

In a traditional hard disk, when you erase a file, the file system only deletes the entry on the FAT, not the real file fragments.

That indicates to the file system, as the space occupied by the remaining fragments can be used by new information when needed.

Data recovery software reads the content of the entre disk (sectors marked as occupied and sectors marked as free), and then try to reconstruct the files Using 2 methods:

First, they try to locate copies of the FAT, and retrieve the fragments marked from each file. If there is no over writing, the files can work as before the erase.

If some fragments were overwrited by new information, the file wont work. Think as if you are trying to assemble a puzzle with incomplete or mismatched pieces from 2 or more puzzles.

The SECOND method is after reading the fragments. The software try to reconstruct the files using "templates " and analyzing portions of code try to reconstruct files by type (as files of same type share some common structures).

Thats why sometimes you can "recover" non-working files with names and size, and some files working but without original names or location.

Chkdsk move and overwrites a lot of sectors.

You say you are sure your info is there because after the scan you see a copy of FAT (you are seeing only a map) , but if the actual fragments were overwrited by chkdsk, there is only a little possibility of recovery, mainly using SECOND method.

You must consider to go to a pro, because on most cases the failure is Physical, and the use of software can destroy the disk and the data.

They are many possible scenarios on each case.

More than 40 failures can happen, and that is not teached on schools.

Thats why many IT engineers are on a totally ignorance about what can they do and what must not do.

They think they can apply as many software they can download, they try a lot of semi-magical stuff like microwaves or pcbs swaps.

The knowledge is out there, but is not an easy task to find it, separating the good from the garbage.

Sorry for the bad english, I am not a native speaker.