SIMPLE SOLUTION 1:
You could purge only from your "off-topic" style forums to 1yr or 6mo or less, keeping the meat of your general topics in tact.
SIMPLE SOLUTION 2:
Your database tables may also have an excess amount of "overhead" (read
HERE and
HERE) that can be reduced, and could significantly lower the overall disk space your database uses.
1) Close your forums.
2) Make a backup of all your forum tables - or your entire forum database.
3) "Optimize tables" or "Repair tables" from either command line SSH or from phpMyAdmin (if available)
From phpMyAdmin:
a. from the server web interface
b. open the table for your forum
c. at the bottom of the table list, click "Check All"
d. next to that, from the drop-down list, select "Optimize table"
e. wait. wait some more. keep waiting until the process is done.
when complete, you'll hopefully see a green section at the top of the page that says, "Your SQL query has been executed successfully."
Now review the disk space that your database tables are utilizing, to confirm the space has been freed; Click on your database name from the top of the page to view the list of tables again. The total size of all your tables will be shown at the end/bottom of the list, on the right.
4) Reopen your forums.
These simple solutions should provide some breathing room for your disks.
