The following tips and suggestions apply specifically to the use of an EXB-120 CHS with Backup.
The slot and drive addressing in the EXB-120 CHS is as follows:

If you install less than four media drives in the EXB-120 CHS, you must install the drives in the first physical positions in the autochanger. For example, if two drives are to be installed, they must be placed in the first and second positions. If three drives are to be installed, they must be placed in the first, second, and third positions.
The EXB-120 CHS has the "element status" feature. Keep the autochanger loaded with volumes to optimize the inventory process.
If you insert or replace a volume in a slot, you must issue the nsrjb command with the initialize element status (-E) option to update the autochanger inventory. If Backup encounters an empty slot during the inventory, it rechecks several times to make sure the slot is truly empty. This can increase the time it takes to inventory the slots.