Are you backing up this backup drive to tape? If so, figure out some sort of schedule where you keepa a couple of previous days worth of full-backups and the related point-in-time backups and delete those that have made it to tape.

If you're not doing such a backup to tape, then I recommend that you start. Having all of your backup information on a single device (set of drives) can be pretty bad especially if something happens to the entire building where your servers and backup devices are all taken out by fire, flooding, or what have you. You should also consider a DR or "Disaster Recover" plan of some sort. Typically and at the very least, your tape backups should be stored somewhere offsite ad quite a distance from the servers they were spawned from.