Full definition
On hard drives, physical bad blocks come from magnetic platter wear, contamination or head crash. On SSDs, they come from NAND cells with exhausted write cycles (wear-out). Logical bad blocks come from filesystem corruption or ECC failure. The drive's firmware maps known bad blocks into a remap table (G-List) to reserved areas. When that table fills up, the disk starts showing user-visible errors. In recovery, bad blocks require reading with specialized hardware (professional imagers like PC-3000 or DeepSpar Disk Imager) capable of multiple retries, different read speeds and skipping bad sectors while extracting the maximum possible from the disk.
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