• data is stored on platters, coated with magnetic material
• address specified by track and sector   (sectors/track ≅ 200, sector size 512B)
• access time
  • seek: physically move the read/write head to the specified track:
  • latency: wait for specified sector to be rotated under the read/write head
  • transfer: read block

Typical cost: 10ms seek + 5ms latency + 0.1ms transfer