Only one thing to add:
When I tell a program to write to a specific sector on a disk, I cannot know anymore that the disk will really write there.

I can only know that it will write the data somewhere.

We tried that in an informatics class: Write data to the outer rim and to the inner rim of the disk. The outer rim should have different access values than the inner rim, for physical reasons (the disk spins).

Our results didn't show that effect. Rather they showed us that modern hard disks optimize the data layout internally, so even when some tool completely erases any trace which could be found by software, you can't know if there aren't parts which remain in a sector the disk began to see as unfit.

Another example: If my disk gets a "dead bit", I won
t see anything of it, if I didn't have some data on it.
If it's empty, the disk will just silently remap the logic bit to some otehr hardware bit, and no software tool will be able to access it.

Long story short: Additionally to recovery programs, a physical check of your disk can uncover stuff which wasn't even reachable by software anymore.

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