Methods and structures for providing on-the-fly head depopulation in 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 allow on-the-fly head depopulation to protect data when a subsection of a storage device, such as a head of surface is failing. When the storage device detects that a head is failing, data may be migrated off the failing subsection into other subsections (e.g., a different head or surface) using mapping features and aspects hereof. Thus, the data on the failing subsection is still available should the subsection or head eventually fail.