If it's a an autochanger, edit the Autochanger resource in the admin tool, select the Volumes tab, and change the lines in the Loaded Volumes, Loaded Barcodes, and Loaded Slots fields.
Be very careful. Delete only the contents of the lines, not the lines themselves. For example, if you have three drives, all empty, you should have three blank lines in those fields.
You *can* mess up your autochanger if you aren't careful.
Example:
Drive one contains full.001
Drive two contains full.002
Drive three contains full.003
*but* drive two is actually empty.
Your Loaded Volumes section would read (using ^ and $ to mark the beginning and end of lines):
[tt]^full.001$
^full.002$
^full.003$[/tt]
You would want to change it to:
[tt]^full.001$
^$
^full.003$[/tt]