Firmware: how to avoid auto update

My scenario is using Garmin Express on the PC (Windows) and connect the EE2 by USB cable. The EE2 then is in mass storage mode and you can see / control files on the device using the Windows File Explorer. Disable all automatic modes of GE.
Settings in Garmin Express:
- do not automatically launch GE on starting the PC
- do not automatically open GE when a device is connected (GE must be running in the background).
- Schedule: Update Schedule -> Never
- do not connect the EE2 to your mobile phone (Bluetooth Off)
In this way you have full control of what is happening, but I realise that while underway this scenario will not fit at all or only partly.

Supposing that in one way or an other the new software update file GUPDATE.GCD was placed on the EE2 in the folder ..\Garmin you can prevent installation of the new software in two ways:
- Pay close attention when starting up your device and always reject the proposal to install the new software. However, there are also reports of the absence of this dialogue and that the new software is installed automatically. Bad luck...
- after each synchronization, rename the GUPDATE.GCD file in the ..\Garmin folder, for example GUPDATE.GCD.tmp. Always pay attention here too - if you forget the rename once, the new software is on the device! After every synch you'll get a new GUPDATE.GCD. If you have already a renamed version you can delete the new one.

What some people suggest - copying an empty file GUPDATE.GCD in read-only mode in ..\Garmin proved not to work for me - that file is always replaced by a valid GUPDATE.GCD update file from Garmin.

If these tips will work too while having the EE2 in full connected mode - linked to the smartphone with Garmin Connect Mobile on it - the smartphone having internet access, I don't know.

Thank you for this tutorial!
I was hoping for a more simple solution as I will need to use GCM.

When using GCM is there a dialogue / option not to install the update when restarting?

Haven't found this option in GCM yet.

That option is not in GCM but on the device. My question was if that dialogue also appears on the device if you use GCM in contrary to using GC Web. My guess would be, yes.