To ensure correct processing of all tape mount management-eligible data sets, the system must determine the original target device class for the data set. It examines the UNIT parameter provided or defaulted on the allocation, and checks that the data set should be allocated on a tape volume.

Change the JCL or dynamic allocation that references an undefined unit name where the data set is eligible for tape mount management, to specify a unit name that exists in the configuration and contains tape devices. If you do not, the system might replace a valid data set with an empty data set, causing data loss.