Hello,
If you are using our built-in Modbus/Bluetooth/DNP3 database, then the location for this file cannot be customized and it holds the data in RAM, meaning that it will not survive the reboot.
If you are using your own script for such purposes, I would recommend not expanding the storage, as the USB drive can still be explored, but it will be seen as another drive, instead of being "merged" into the router storage.
Rebooting the device while the storage is expanded should not cause any issues, however, storage expansion is meant more as a stationary solution, as accidentally unplugging the USB drive will most likely cause firmware corruption.
Best regards,
DaumantasG