I am giving one general Example
Just consider your hard disk size is 80GB
From 0 to 10GB you saved PICTURES
11 to 30GB ----- PDF
31 to 60GB ----- MP3
61 to 75GB ----- TXT
If you try to save one 10GB movie then it shows No enough memory
It will not overwrite your any data because these files (PICTURES,PDF,MP3,TXT) are now protected by your file system.
Now you delete PICTURES, at that time that picture storage location will be detached by your filesystem ( That means now it is not protected by your file system, anyone can write here)
After deleteling also you can recover your PICTURES (as RedGrittyBrick said) that is because only logical address are gone, but that data are still in your hard disk in the form of bytes.
Now you saved your 10GB movie
At that time from 0 to 10GB PICTURES are replaced by this MOVIE
Now if you try to recover your PICTURES then its not possible
That is because it deleted from your dard disk too.
If memory location 20GB-22GB free and 50GB-58GB free then your file get split into 2 part
half part saved 20-22 and half under 50-58.
If your files saved part by part in diffrent location then your computer becomes slow that is because to read a file the header (hard disk read write header) will be moved to several place and it takes time.