FSD files - can I delete them? These seem to take up loads of space.  Can I safely delete them or will it cause problems?
Those files are cache files for syncing changes between OneDrive for Business and/or Sharepoint shares.

Is it safe to delete?   Yes.

However, they'll just be recreated as you sync back and forth your OneDrive folder(s).

However, there's been a fix for this for some time now - make sure you update your OneDrive client and that you enable Files-On-Demand