Shingled magnetic recording (SMR)
Letting the write tracks overlap like roof shingles — denser data, but every rewrite tears up the neighbouring track.
The write head is wider than the read head, so SMR writes tracks partly over one another like roof shingles and squeezes in more tracks per inch. The catch: rewriting one track smears over the next, so the drive must read and rewrite whole zones on every change — excellent for archives, dismal for random writes. The bookkeeping happens either in the drive (drive-managed) or on the host (host-managed).
The technique reached archive drives around 2013 and caused an uproar in 2020 when Western Digital quietly slipped SMR into NAS drives, where random writes are exactly what dominates.