I'm getting this error continually on a drive that has about 3x more space than the main storage drive.

zimbramon[23069]: 23069:err: Disk warning: Z1 /backup on device /dev/xvdb1 at 90%

How do I tell Zimbra to recycle the space and keep it below the 75% threshold (Backup Minimum space setting is 25%) rather than just complain about it all day?