Just value it at $20 CAD (maybe use $15 US) and be done with it. If you try to mark it with special instructions along with a high value you'll just be asking for trouble.

You can still include something like "Temporary import" or "Contents will return to US"

The other alternative is to pay the taxes (you won't be charged any other duties) and then try to get them refunded. You won't likely be refunded any handling or brokerage fees however unless the drive is being sent via postal service.