Why does deleted data stay on a HDD once written, waiting to be overwritten, as opposed to being removed when requesting deletion?
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The primary reason is speed. During a normal delete the only thing actually deleted is the pointers to file, the data is still there unless you specifically trigger some process to overwrite the data.
Interestingly for quite a while computer forensics folks have know that they can recover data from magnetic media (like a traditional hard-drive) even if its been completely overwritten. Presumably this is possible because of subtle differences in the magnetic ones and zeros that are still left behind in the analog signal. So good hard drive wiping software will overwrite data with consecutive ones and zeros as with as many as five iterations.
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