shred overwrites file data before removing it, which can be useful on some traditional magnetic disks. It is not a reliable guarantee of secure deletion on every modern storage system.
Why SSDs change the picture
SSDs use wear levelling and internal remapping, so rewriting the same logical blocks does not necessarily overwrite the same physical flash cells. Copy-on-write filesystems, snapshots, journaling, backups and remote replicas can also leave additional copies.
Better approaches
- Use full-disk encryption so discarded data is protected by key destruction when appropriate.
- Use the storage vendor’s supported secure-erase or sanitize method when disposing of an entire device.
- Review snapshots, backups and replicas if the goal is to remove all copies of sensitive data.
shred is still a useful command to understand, but do not treat a successful run as proof that data has vanished from SSDs or layered storage systems.