HODLER Wallet is always offline by default but in case of broadcasting transaction where being online is a must, you can quickly disconnect connection by using button on the right side of device (This bottom will shutdown screen, and take device in something like "Airplane" mode). USB Connection is fully controlled by wallet and the port will remain only for charging batteries. Masterseed is stored in encrypted partition but at this moment using SD Card to make seed backup is not excluded in our dev's roadmap.