Someone suggested to me that perhaps Laserfiche volumes work like a WORM drive--once you store a file, you can never get the storage back. That doesn't make sense to me so I trust someone here can prove them wrong.

Stating on the record, in an official capacity, that said suggestion is incorrect.

Files logically deleted from a repository are "marked for deletion", immediately disappear from the repository, and then Laserfiche Server (attempts) to delete them from disk during the daily internal maintenance window (as Jason notes above).

Check event logs to see if maintenance operations are failing for some reason.

For example, if the Laserfiche Server service doesn't have "Delete" permissions on the volume directories, it can't clear out the underlying files even though they're marked for deletion.

Another potentially easy way to reclaim some space (if you haven't done so already) is to copy older audit logs elsewhere and then delete those files from the disk that's almost full.

If you have any sort of secure-enough SMB-accessible file storage anywhere on the network, you could roll the volume over to there. Use the UNC path and make sure the Laserfiche Server service identity has the necessary permissions to read/write/modify/delete to it. If LFS is running as Local System, you can set the share permissions on the AD Computer object for the server.