1. An apparatus comprising a solid state storage device in which a solid state storage in the solid state storage device comprises erasable blocks each erasable block comprising a plurality of data write locations, the apparatus comprising memory and control logic adapted to: store input data in successive groups of data write locations, each group of data write locations in the successive groups of data write locations comprising write locations in a set of erasable blocks in each of a plurality of logical subdivisions of the solid state storage; error correction encode the input data such that each group of data write locations contains an error correction code for the input data in that group of data write locations; maintain, in the memory, metadata indicating a location of the input data in the solid state storage; maintain an indication of validity of data stored in each data write location; prior to erasing an erasable block, select an erasable block from the set of erasable blocks for erasing in dependence on a number of write locations containing invalid data in the group of data write locations which contain write locations in that erasable block; and recover valid data from each group of data write locations containing write locations in that erasable block thereby forming recovered valid data and re-store the recovered valid data as new input data.