Sigh,
Right most hard drives have spare sectors for error correction, and redundancy.
They have a set number in case where data resides on a sector of the platter of the hard drive becomes damaged.
or starts to become unreadable.
It copys the data to a new sector then flags the damaged sector not to be used, written to.
Once you use up the spare sectors as the drive or platter begins to fail.
You get reallocated sector count. it means 48 sectors of the drive are damaged or unreadable.
Once you hit 50 you have no more sectors left. and the information can not be copied or remapped to save it.
The data gets corrupted and is lost forever.
In short the drive is starting to fail on parts of the platter that hold information and cannot store the data leading to corruption.
It is saying buy a new drive before information is lost.
If you keep using the drive then the errors will become more frequent, and more evident over time.
Sometimes it can be an error in the file table allocation of the drive.
That can be fixed by software., or windows it`s self.