1. A method for storing data in first and second write-once memory devices, the method comprising: (a) sending, from a first write-once memory device to a file system, an indication of a minimum number of memory cells of the first write-once memory device that can be written into during a write operation; (b) with the file system, writing data into no less than the indicated minimum number of memory cells of the first write-once memory device; (c) sending, from a second write-once memory device to the file system, an indication of a minimum number of memory cells of the second write-once memory device that can be written into during a write operation; and (d) with the file system, writing data into no less than the indicated minimum number of memory cells of the second write-once memory device.