Backing up saved game files on Xbox One

I also use the Game Drive Hub for Xbox by Seagate it's 8 TB of pure gaming goodness! I highly recommend, you can also back up your entire gaming library with this bad boy.

I back up my files and copy my games to an external drive from seagate. Try this link for information

https://support.xbox.com/en-US/xbox-one/accessories/usb-port-use

Try go in Settings>System>Kinect & Devices > Storage and try see if the configurations is correct there

If you backed up the save file to a flash drive or the Cloud, you could boot up the Xbox 360 game on another console and use the in-game process to transfer it from there, but aside from that there's no way to export the file to the Xbox One version.

You'll get a message that says 'Which one you want to use' you get this message when your console has newer or different locally saved data than what’s saved in the cloud, usually after you play offline. You’ll be given the choice of which saved data to
use, the locally saved or the cloud saved version (and you’ll be notified which is newer). Pick the “save” you want to use and continue.

So you can, play online for starting the save but you can play, offline and as soon as you re-connect, to the internet and launch the game you'll see the message while it syncs.