Filemaker 12 will handle external storate natively. To change this for existing records, determine where the files will be stored. Then go into manage database and select the Container field. Then click the storage tab. At the bottom under the heading Container click the checkbox for "Store Container data externally". You have the option of Secure or Open. For video, I would use open storeage.
When you click ok to close the Manage Database window, FileMaker will ask you if you want to convert the records now. With 15 tb of video, this will probably take a while. Sorry I can't predict how long but possibly hours or days????. So possibly do the conversion at point when the DB can be off line for a while. I sat through a FileMaker demo of this at one point. If I remember this correctly you can make the change and tell FileMaker not to convert immediately. It should start putting new records into the external storeage and you could select a time to convert the existing records.
To get a feel you could set up a test DB with a few records and convert them from internal to external storeage. I have actually worked on a couple where it got converted to external and back to internal so the records would images would be available on an iPad. These were images so a lot less data. In this case the conversion was a minute or so. Not a whole lot of records to do.