A mass storage memory system and method of operation are disclosed. The memory system includes an interface adapted to receive data from a host system, a plurality of memory die and a controller, where the controller is configured to read or write data synchronously across a plurality of die connected to different channels based on a first command, and to read or write data asynchronously and independently in different die in the same channel based on a second command. The controller may program data in a maximum unit of programming for a single memory die. The controller may be a plurality of controllers each configured to select which die of an exclusive subset of die to write data based on characteristics of the die in the subset. The plurality of die may be multi-layer, and multi-partition per layer, flash memory die.