An exemplary memory subsystem with fault isolation comprises a first data bus routing data groupings in a lower 72 bits to a first memory expander, and a second data bus routing data groupings in an upper 72 bits to a second memory expander. A first memory module receives all of the data groupings in the lower 72 bits of each memory expander. A second memory module receives all of the data groupings in the upper 72 bits of each memory expander. A failure in any one or more bytes in an ECC word indicate failures in the computer memory system.