Yes, you should not use a reference.
Bit more explanation: External storage doesn't use a reference because a reference is a file path. FileMaker will manage the file paths for you as part of the feature. Everything goes into the RC_Data_FMS directory inside each database folder:
From there, it follows the path you specify in the Options. It's generally a good idea to include the ID of the record or some other unique identifier rather than just dump everything in the same directory when using open storage, since it's possible to overload the OS's ability to index a folder with lots and lots of files in it.
Or, just use secure storage and let FMS handle the file paths. That's what I generally do. There's no need - ever - to touch the external assets with anything other than FileMaker, so secure storage actually helps protect them from inadvertent damage.