If you have 6 filesystems backing a sequential access file storage pool, and you remove one filesystem, TSM cannot calculate free space properly.
Instead of looking at the free space of the remaining filesystems, it take the total space of the filesystems, minus the volumes in that device class.
Since there may still be old volumes in the “removed” directory, it considers the device class 100% full if everything currently existing cannot fit into the remaining directories.
Note that removing a directory from a device class does not invalidate the existing volumes in that directory. So long as the directory is still accessible, the volumes will be usable.
This is a problem when you want to reduce a filesystem but not migrate 100% off of it, as there is no other way to tell TSM not to allocate new volumes in that directory other than to remove that dir from the device class.