Methods and structures for dynamic density control to improve reliability of a dynamically mapped storage device. In a dynamically mapped storage device in which all user supplied logical blocks are dynamically mapped by the storage device controller to physical disk blocks, features and aspects hereof provide for dynamically altering the recording density of user data stored on the storage device. So long as the physical capacity utilization of the storage device permits, new data stored on the device may be stored at lower density to improve reliability in reading back the recorded data. Further features and aspects hereof may reduce the recording density only for data deemed to be critical. Radial (track) density, longitudinal (bit) density, or both may be dynamically controlled to reduce recording density. As physical capacity utilization increases, data previously recorded at lower density may be migrated (re-recorded) at normal higher density.