For Inspire 1 controllers with Firmware before v1.3, the update must be done via USB. However, any newer versions of the firmware are updated directly from DJI Go. Because of this issue, the controller needs to be updated to Firmware v1.3.0.00 using USB. Then, the controller can be updated to the current version using the DJI Go app. Please download this Firmware file to get started: Inspire 1 RC Firmware v1.3.0.00
After you successfully install v1.3.00, then you can update the aircraft directly to the v1.8.1.00. The aircraft is still upgraded using the SD Card in the camera. But the RC will upgrade via the GO App.
Take a look at our article on updating the Inspire 1 Firmware using the DJI Go app: https://support.grescouas.com/support/solutions/articles/31000127392-updating-the-remote-controller-firmware