Is there a reliable up-to-date way to find media that resides on the server but is not used in the WP site?
In short: no, not as far as I’m aware.
I have found a number of plugins, but all are very old, with lots of negative comments to the effect that they do not work.
The reason for this is that what you want to do is incredibly difficult, especially to do as a plugin or through automation. Consider this:
- Uploaded images get stored in a YYYY/MM/image.jpg format
- They also may get stored in multiple sizes
- Their only reference to this file in WP is in the attachment meta data
- Any reference in posts etc is through the attachment ID
- This ID can be in several difference places
So, you would have to have a plugin that can match a file to an attachment ID, and then go through every possible field – and in a multisite installation every blog’s version of those fields – and look for a reference to the attachment, and maybe then you would be able to have a result that you could use.
I can’t really think of an easier way, but that process could probably be made a little easier with automation – it’s just not something that you’ll likely find a good plugin for.