Yes it was unmounted. I ended up using a python script to delete the files.
In case anyone else needs to do this and finds it useful, I took the script from here -> https://github.com/JackPo/gdrive_orphan_cleanup/tree/main
For my case (no other files in Google Drive apart from the chunks), I just needed to amend line 99 of the code to
if not item.get('shared', False):