I solved this by just moving all the Container image files into the same base-directory as the FMP13 file. My slide scanner software was putting it into it's own default folder. I tried adding that folder-path to the Manage Containers but that did not work. So, I deleted that path entry and moved the files via Finder which, I reckon, is what I should've tried first.
The files produced by the scanner can't be directly added to the folder specified for external storage. You still need to insert them into container fields so that FileMaker can then copy them to the external storage folder from the container field.
There are several options that you might try for using FileMaker to insert all the files in the Scanner's folder into container fields in FileMaker. If these are image files, Import Records | Folder can import the entire folder contents in one batch operation.
FM kept rejecting my attempts to insert images into the container field until I put the image-folder into the base-directory folder making it a sub-folder of the base-directory folder. Just adding the image folder to the Mange Containers dialogue was unsuccessful, as in the uploaded image. I wasn't trying to get the scanner to insert images, directly into container fields. They require editing and renaming before inserting them into their corresponding record, regardless.
Using Import Records, would I not still need to rename the image files so FM can match the images to their correct record? How would I accomplish that in a batch-operation? I have hundreds of slides to scan and add to the database so, I'd love to use batch-operations whenever possible.
Why would you need to rename the files?
When you use Import Records | Folder to import files into container fields, there's an option that lets you specify a text field in the same table to receive the file name of the imported file. You should be able to use such a file name field in order to match the imported images to the correct records in your table.