My bet is that there is some very small picky difference between the filepath you are computing and the actual file path. You might download the exploration file found here, Insert a file from that folder with the "by reference" option specified and then use the calculation fields in this file to compare the file path extracted from the container field to your computed file path.
Perfection, Phil. Thank you.
Issue was: Needed /Volumes/ when creating the folder but not when specifying the path.