The z/OS® media server cannot allocate the specified device during a ZOSMEDIA library request because the UNIT value in the device class that is associated with the volume mount request specifies a device address that is already allocated.
The Tivoli® Storage Manager server fails the operation.
Verify that the MOUNTLIMIT parameter value in the device class is 1 if the UNIT value specifies a device address. Additionally, you can update the MOUNTRETENTION parameter value to 0. This will ensure that the mounted volume is dismounted after each use.