Hi Pete,
We had such a situation once, some files were failing to drain for some reason (one was outdated .xsmap files
You can inspect the drain situation with command eos fs ls -d, it will give you the interesting information of the number of files remaining. Then I think that the good way to extract the list of the files is eos fs dumpmd <fsid> --path
So you can then investigate why these files fail to drain, potentially converting them manually so that they are moved elsewhere.