I am going from Canada to Mexico but I don't want to be dependent on in-flight entertainment. Thus I got seagate wireless plus so I can stream to my iPad. I couldn't find any policies I can use this device during flight or not. At very least I am trying to determine if it is worth asking the flight company?
No, the rules are about any transmitter or receiver, regardless of whether it comes under the specific category of phone/tablet/laptop etc
If anything your device is worse as it transmits at a higher power and range than a receiver.
Airlines provide WiFi onboard and invite you to connect to it (for a fee). To connect to WiFi you must have the ability to transmit (internet requires two way communications). If your device is conforms to FCC power limits on the 2.4GHz bands (or whatever your particular jurisdictions regulatory body is) then your device is no different than any other WiFi device on the plane. They are all transmitters using the same maximum power.
If your airplane has WiFi onboard and let's you connect computers to it then there is no technical problem with using your device.
Of course, if a crew member tells you to turn it off, remember you must comply.
The intention behind is to isolate the electronic devices of an airplane from electromagnetic radiation.
Modern airplanes are pretty well protected at the moment and they don't have any issue of isolation.
However, one thing that is key for authorities is flight safety. So it is better to have stronger restrictions that making any mistake.
So, in order to be able to use wifi on the airplane, you need to have, first the airplane modern enough and the airline shall be authorized by the authorities.
Finally, in any case those radiation devices should be turned off (or in airplane mode) when flying in take-off or landing stages (so the wifi of the airplane does).
My recommendation: ask the airline directly and get information about the airplane you are flying in.
The airlines will be primed to say No. But you will hardly be the only one on board with an iPad so it's unlikely anyone will notice. Turn it off if the plane suddenly pitches nose-down at the beginning of your movie.
However, I question the need. I have a 32Gb iPad and there's absolutely no way I can watch a full load of video on my entire vacation, never mind the flight at each end.