When a file is deleted, the first character of its name in the directory listing is changed to a non-valid character that acts as a flag to the OS to no longer show it in directory (folder) contents listings. The disk clusters that were allocated to that file in the file table are marked as available for the OS to use to store new files, so far their contents is unchanged. So as long as those clusters don't chance to be used again for another file, and have their contents overwritten, file recovery software can readily re-instate the previous condition and hence recover the file.
A similar condition occurs when a hard drive is formatted. No change is made to the data contents of the drive. The format will reconstruct the file system (that is the structures that catalog the identity and location of files on the drive), which is a quick format, and a full format will also perform a basic check of the data integrity of the drive. If the drive is re-partitioned involving changing the size of partitions, some of the filesystem structures will move location and change in size, which makes recovery more difficult, but still feasible. The remainder of the data that was on the drive remains there, this is what software like PC Inspector and GetData Back utilises to recover drive contents after a format.
The situation becomes rather different once an erasing utility has been used. From manufacturers zero-fill utilities, which reset the RLL sequences for every bit on the user data area to read zero, to utilities like Eraser, which cycle the bit values of every location up to dozens of times, the aim is that data is changed to no longer contain the information it once contained. So there is nothing that normal drive access will return to recovery software, and forensic recovery techniques must be used to attempt to determine what data each drive sector contained before
the erasure. This immediately becomes time-consuming and expensive, so unless you suspect that someone will spend thousands of dollars to try to get that information, a secure wipe like eraser's Guttman cycle is quite adequate (maybe even overkill).
You can then re-load the drive with whatever OS and applications belong with it before you sell it.
Edited by Platypus, 27 December 2006 - 08:33 AM.