It is quite normal form the device name to change if a drive disconnects and then reconnects as the sdX type names are assigned dynamically at the Linux level, so you can never assume they stay constant (this is one of the reasons Unraid identifies drives by their serial number and not the sdX type device Id). This will cause problems if the USB drive is used in the array as the core Unraid system is not hot-plug compatible but if the disk is being used as a Unassigned Devices then the UD plugin should handle this OK.