What is the most efficient way of taking over a drone?

SPOOFING
You locate the owners persona, look at it. Make a spoof command to the drone. Orders drone to switch subscription list to your own – you now own a drone.

HACKING - Drone
Hacks the owners commlink. Locate connection to drone. Hacks drone. Edits the drone subscription list.

HACKING – Commlink
Hacks owners commlink. Edits the subscription list so that the commlink only answers to my commlink. Switch subscription list on all drones to my commlink. Copy all information to my commlink, erase all info on subverted commlink.

Final action: Spoof datatrail.

With these three tactics you can take over a drone. Personally the third option seems the most quick and simple one.

Have I missed something apart from taking into account security IC and agents?

You forgot: hit drone with electrical weapon, put burned out drone on back of truck, take home, replace burned out components.

Or better yet:

Hack drone production line, insert 'me== the boss' into the logic of all drones produced at the factory.

How this for a run idea: Strato 9 drones are very popular with Lone Start. Pose as/hack the manufacturer and issue a 'security patch' for the Stato-9 series of drone. Patch includes the 'me== the boss' line. Of course, such tampering would probably be eventually detected by the LS security folks, if you handn't tracked down the important ones and put 'me==the boss' psychotropis IC on their commlinks beforehand.

i vote hacking, its the onyl way to get all of the drones he has in one fell swoop