failed backup

keep getting failed backup message:

the b/u disk image "/volume/----time caps/--powerbook G4 17". sparsebundle is already in use.

dashes refer to my title of my backup. Started happening in April. Reference is to my old powerbook. I've had desktop for 3+ years. Don't recall using my time capsule to backup the powerbook. had separate backup for the powerbook.

can someone help me troubleshoot, one elementary step at a time? thanks.