A) I have to gain access first.
If you check the book you will notice that Control Device have an access level of: "User/Admin (can vary with action chosen)"
So yes, you first need to gain access on the device / the network the device is linked to.
B) After that I have to take one major action to take the Control Device action and get sustained control.
C) I can now use the device for anything I have sufficiant access.
It depend on the device.
For
devices that have a test associated with them (such as drones and vehicles that have Piloting and Engineering tests associated with them) then, yes, you now have sustained control:
"While you have control, you can use the device as if you were the owner using a standard skill test..."For other devices that have no test associated with it (such as opening a maglock or sending the elevator down to the lobby) you
instead use the Control Device action (which is resolved as an opposed Electronics + Logic vs Willpower + Firewall test)
every time you wish to do something with it:
"If there is no test associated with the device you wish to use, use Electronics + Logic vs. Firewall + Willpower. "what's the cost for using the device (e. g. opening a Maglock, firing a drone)? Is it another major action?
It is resolved in two different ways.
For the maglock you take the Control Device major action (Electronics + Logic vs. Firewall + Willpower) to open it and then the Control Device major action again (Electronics + Logic vs. Firewall + Willpower) to lock it.
For the drone you take a Control Device major action to gain sustained control over the drone, an Attack Major action (Engineering + Logic vs Reaction + Intuition test) to fire it and a Use Skill major action (Piloting + Logic vs Perception + Intuition) to sneak pass a guard with it.
In the German rules, if there is no test associated with the device you wish to use, it's made with Cracking + Logic (instead of Electronics + Logic). Which one is right?
It was listed as an Illegal Electronics + Logic test in core but it got explicitly changed in the first English august errata to a Legal Electronics + Logic test.
This mean that if you are already made it inside the network then you can open up a maglock (without getting the attention of GOD) while if you are on the outside you have to illegally use Spoof Command to instruct the maglock to open itself (which might attract the attention of GOD).