its like when you delete files from your PC it does not actually get deleted, its still stored inside your HDD.

It only gets deleted if other files overwrites that location of the HDD.

So if i have 100 MB HDD, i delete 50MB movie, O.S will show 50MB free, but that file is still present in your HDD.

Now if you copy another 50MB movie into same drive, new movie overwrites the old file present in HDD and now its impossible to get that file back.