I believe the nano only looks for the imu at startup, so if it isn’t visable to it then, it won’t keep looking for it. I had similar issue but in reverse, i.e. if powered by usb, the nano wouldn’t necessarily see the imu, but if powered up externally before plugging in the usb it works ok. The solution would be to configure specifically for your imu, and make the nano keep looking until it’s found. I’m not sure if this issue is potentially shared with Panda.