This is the base page of the section of data recovery software.
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data recovery looks at what is essentially a scattered mess and tries to make sense of it. sometimes it is not always possible to do a complete job, as is the case with Undelete programs - some of the data may have been overwritten and therefore no longer accessible. your greatest chance of recovery is when you have done as little as possible to the disk/media since the catastrophic event and you are trying to recover as soon as possible.
for this reason, you are better off not trying to boot the OS, and leave the recovery to a computer forensics expert, or ro try to mount the disk offline to another computer and attempt to recover the disk with various recovery software. some will work better than others for different situations. I am still building up a collection of software.
some recovery software, such as CGSecurity's PhotoRec and PC Inspector, use file extensions or file signatures to determine the type of content in the file. so it will only recover a small number of files with certain file extensions. I have had no good success with PhotoRec. I end up with a bunch of numbers for filenames all piled into 1 directory. no folders.