Deleting is not erasing
Dragging files to the trash removes the reference, not the data. For years, proper disposal meant overwriting every block on the drive, sometimes several times.
Solid-state storage made that unreliable: wear levelling moves data around, so an overwrite cannot guarantee it reached every cell that once held your files.
Why encryption solved it
Current phones and laptops encrypt storage by default — FileVault on Mac, BitLocker on many Windows PCs, and file-based encryption on iOS and Android.
When a device is erased, the encryption key is destroyed. Without the key the remaining data is meaningless, which is why a modern factory reset is both faster and more thorough than the overwriting rituals it replaced.
Account locks are a separate problem
Erasing data is not the same as unlinking an account. A device still tied to its previous owner's account can be unusable to anyone else:
- Apple devices: Find My must be turned off, which removes Activation Lock.
- Android devices: the Google account should be removed before the reset, or Factory Reset Protection can block setup.
- Windows laptops: a firmware password or BIOS lock will stop a reinstall.
A reputable seller clears these before listing. It is worth confirming, because the fix usually requires the previous owner and nobody else.
What to check when a device arrives
- It boots to the first-run setup screen, not someone else's desktop.
- It asks you to create an account rather than asking for one you do not have.
- Storage shows as empty in the operating system's own storage view.
If a device arrives with someone's files still on it, that is the clearest possible signal about how it was processed. Send it back.