Abstract: A virtually addressed multilevel mass storage system (MSS) has error recovery and definition procedures and apparatus for controlling and enabling recovery from error conditions in an upper storage level. A plurality of possible error conditions in an upper level gives rise to errors in destaging data signals to a lower level, plus possible overwriting good data with data in error. A first such error condition is a data error detected during a destage. A second such error condition is repeated upper-level equipment (not data) errors. Both errors make data integrity of the lower-level suspect. Corrective action for a plurality of errors includes coordination with a host computer, reconfiguration, destaging data in error after precautionary steps, and preserving data in error at the failing upper level unit which is used in virtual mode except for the portion yielding the error condition.