TSM Differentials and Expiration

Hi,

We are running TSM 5.5 and have been running NDMP Backups for our EMC Isilon NAS.
Due to the amount of data that we have (over 100TB) we are unable to schedule in a full backup due to time constraints.
Our original full backup took around 6 days to complete with 2 dedicated Tape Drives.

At present, we are solely running Differential backups and are quickly filling up the tapes.
Does expiry take place on older Differential backups, or does a Full need to take place to implement this?

Below is our copygroup.

Policy        Policy        Mgmt          Copy          Versions     Versions       Retain      Retain
Domain        Set Name      Class         Group             Data         Data        Extra        Only
Name                        Name          Name            Exists      Deleted     Versions     Version
STANDARD      ACTIVE        ISIMC         STANDARD             2            1           30          60

Thanks