Flash Translation Layer
The controller layer that maps host-visible logical block addresses onto physical NAND flash pages and erase blocks.
NAND flash does not behave like a disk that can overwrite arbitrary sectors in place. Pages are programmed while erase operations occur at larger block granularity. The Flash Translation Layer (FTL) hides that mismatch by mapping host-visible logical block addresses onto physical NAND locations.
That mapping can move over time. Garbage collection, Wear Leveling and error management may relocate data while the host continues to address the same logical block. A block becoming unreachable at the file-system level therefore does not prove that every older physical copy disappeared immediately.
The forensic and security consequence of that boundary is developed in SSD and NVMe Media Sanitization and Cryptographic Erase.