Looking for a quick way to wipe free space on any volume?
The dgl wipeFree command in diglloydTools does so efficiently without any need to erase the drive (a full erase/wipe is always better if the drive is to be sold, but this is not viable when/if the drive is in active and continued use).
At present, the wipeFree command is command-line (Terminal) only. There is also the wipe-files command which erases file and/or folder contents and metadata.
As it turns out, my most frequent use of the wipeFree command is to wipe free space on Disk Utility disk images so that they compress down to the smallest possible size (e.g. when I zip compress one for downloading).
Note that for SSDs, wiping really cannot truly be done in a security sense due to block remapping internal to the drive. In this case, use disktester recondition command (available in GUI) with total writes exceeding the drive capacity by at least 10% does the job, so that the SSD has to “turn over” all its internal blocks.