The Content Management Starter Solution is not set up to use External Storage as it stands. In other words, any container data is embedded in the file itself and it will become very big very quickly.
I suggest you start by changing the storage setting for the File Container field to "Store container data externally". This way, FM will create its own storage directory—effectively a copy of the original file you drag in there.
It is worth noting that while this document Storing container field data externally | FileMaker indicates that external storage is possible on different drives (mounted volumes), many users on forums have indicated difficulties with that. This thread indicates a solution in FMS13 but for windows only http://forums.filemaker.com/posts/8229935dcd
If it is possible it would probably be wiser to keep the external storage on the same hard drive.