this post was submitted on 24 Dec 2025
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But if I fill a drive with nonsense data, whether SSD or HDD, shouldn't it be forced to write such data to all possible locations, thus overwriting the original data? Is am I misunderstanding something more fundamental about how this type of storage works?
@Logical Filling an SSD with zeros only affects the logical address space visible to your OSβit doesn't force the controller to erase every physical block. The old data remains in unmapped or retired areas until (or unless) the controller decides to erase it later, potentially allowing recovery with specialized tools. Some SSDs might even optimize by not physically writing zeros if they detect a full block of them, simply marking the space as erased without touching the hardware.
See what I'm still not getting though, is how there can still be unmapped or retired areas, if the drive has been filled with (meaningless) data? Let's say it isn't all zeros, but random data instead. Are there more physical blocks than is represented logically by the adress space exposed to the OS?
On big flash memory you typically have more memory on the chips, than ia presented to the OS. Flash has significantly less write cycles, before the block breaks, so the controller monitors the health and won't use it anymore when it will soon fail. Instead it uses a block from its unused extra space. (Details might be different, I'm not sure about that). This way the lifetime of the SSD is significantly improved. SD cards do the same, I think.
So the data in the retired blocks will remain and cannot be overwritten by the OS. If they are encrypted and the keys deleted, that won't matter
Okay, that makes a lot more sense then. Thanks!