Not sure if this applies to your unit or not but I saw this on another forum.

******Extremely Important ******
Please take note Helix G1 and G2 users:
There will an official bulletin from Humminbird probably later today or perhaps tomorrow, but I wanted to make sure you were informed as soon as possible.
We will also update shortly the Humminbird website (software download area).
Please take action and update your processes and inform all your relevant staff, dealers and customers.
This information affects HELIX G1 & G2 units
If a unit is running old software (version 1.3x or earlier) and an attempt is made to update the unit to the latest software release (version 1.900) the unit will FAIL and be rendered unusable (it would need to be returned to Humminbird USA to be reflashed).
The solution is to firstly update the unit to software version 1.640 (available on the Humminbird extranet), and then you can update to version 1.900 without any problems.
Units that are already running 1.64 or newer are not affected and can be updated directly to 1.900.