To reduce disk space, you will need to reduce your retention periods for the reports.
From Configure tab, select Network and then select the DC. Next, goto the Configuration tab, then reduce the # of hours in the retention period for all three columns (Class, Partitiom and Link). Caution: All three columns need to be equal. For example, if you select 25 hours for RAW - all three need to be set to same value.
Please understand that reducing this value will change what the reports show... For instance, for a day report, it is highly recommended that you choose 25 hours for a complete day and for Day select at least 2 days and a month report should reflect 2 months so that you have a complete time period.
These new retention period will take affect immediately.