a block correlation table includes block addresses of unusable block portions in an irreversibly writeable memory and includes addresses of associated substitute block portions in the irreversibly writeable memory . a request for data stored at a logical address is received from a host processor . a physical address in the irreversibly writeable memory is calculated from the logical address using a fixed mathematical relation . the physical address is compared with the block addresses in the block correlation table . when the physical address does not match any of the block addresses in the table , the irreversibly writeable memory is referenced to read data stored at the physical address , and when the physical address matches one of the block addresses in the table , the irreversibly writeable memory is referenced to read data stored at the address of its associated substitute block portion . the read data is transmitted to the host processor .