No experience of tablets but if they have anything in common with "smartphones" they will default to internal memory, and bar the use of SD memory for apps and suchlike (presumably for a laugh). I know when I found my SD card not being used my investigation indicated tthat if I wanted to put apps there I'd need to "root" the handset first !
Data for video is usually not a problem though. Maybe the app used to create the video has a setting that needs the box ticked ? Meanwhile try looking to see if you have, or can download, some file app that allows you to cut & paste files from one location to another.