Hey Ocean,
Great question. Check out our documentation on resetting the audit log here.
If you're interested, here's the explanation: The audit log keeps a record of past changes to allow MirrorSync to merge conflicts, however it can get huge, up to a maximum of 1 TB. The solution is to use the configuration client to reset it. Log in, right click the configuration, mouse over Reset SyncData and click "Only AuditLog".
Despite its size, the audit log isn't critical to MirrorSync's function; you won't need to replace your spoke or even expect much longer syncs after you wipe it out. You may not even notice the downside: that MirrorSync won't be able to merge some record changes after you delete the audit log. However, after resetting this problem will quickly resolve itself; after a few days of normal use it'll be as though nothing happened, and you'll have your ~80GB of space back.
Hope that clears things up!
- Adam