I have an idea of what may have happened. Is it possible that the form was completely filled out with all required questions completed, saved as finalized, and then you edited the form? If you edit a finalized form, you can't save with the diskette unless all required fields are filled out and validation conditions are met. If you want to save as draft, you have to jump to the final screen of the form and uncheck "Mark form as finalized".
All of this is a bit strange. We are intending to make big changes to this area of the app to hopefully simplify the notion of drafts and forms ready to send. We're looking for users to interview so if you've run into issues in this area of the app, consider getting in touch: Talk to the ODK team about form finalization and edits in ODK Collect