1. A computer system, comprising: a CPU; a memory controller coupled to said CPU; and a plurality of slots coupled to said memory controller in which hot pluggable memory boards can be inserted; wherein, if one of the slots is currently occupied by a first memory board and a second memory board is inserted into the other slot while the computer is operating, the memory controller will automatically copy the contents of the first memory board to the second memory board and then enable a mirroring mode of operation in which write cycles to memory are performed to both memory boards.