Transferring save data for Sea of Thieves from an internal hard-drive to an external hard drive (Xbox One)

Sea of Thieves does not store savegame data locally. Everything is stored on servers. I can play on my Xbox, and pick up right where I left off on another Xbox or my PC, and vice versa.

The only exception to this might be configuration type of data (e.g. graphics settings, keybinds, etc) and as far as I know, those types of files are (basically) impossible to manually manage and move. At least, its impossible using a stock Xbox with built in GUI. Its probably possible with some modification or a custom ROM, but that is far past the scope of this question.

If you are deadset on getting your savegame data onto an external hard drive (for "reasons") it is impossible without "hacking" and breaking the EULA. MS specifically prohibited and prevented this back in the Xbox 360 days since it could be used to develop cheats and obtain achievements.

I wholly believe your motivation is innocuous, and in that case, I cannot imagine a need to get your savegame data onto an external hard drive. If you provide some additional details regarding your end goal, we may uncover an X Y Problem