Abstract: In a non-volatile memory system, the controller maintains in its volatile memory two free block lists for the assignment of memory circuit blocks when writing user and system data. Copies of the free block lists are maintained in the non-volatile memory. While allocating blocks from a first of the free block lists, the controller can update a second of the free block lists as part of a control sync operation preparing control data stored in non-volatile memory. This allows the memory system to operate in a non-blocking manner during the control sync. Once the second free block lists is prepared and the control sync completed, the second block can subsequently be used for block allocations and a control sync operation can be done to update the first block.