Hi @Peter-Gamma
I am a bit dubious as what is your need to have a server on a handheld device.
Servers are used for higher capabilities, it can be higher storage volume, more processing power, large RAM size or large bandwidth...
Using a smartphone or a tablet as a server is not something I would advice, it would be better to keep a mobile device as a client (it will also save battery).
From what I could understand of your various messages about SD cards, SMB and now Minecraft ?! It seems you're running all over the place.
Given your previous post, I imagine you want to be able to backup your phone in case anything went wrong. You also want to easily swap device.
For the syncing there are already existing apps to fulfill your need either syncthing or nexcloud client (I think there are a couple in the open store)