So, I've seen this question come up a couple of times and the response is generally the same and it seems that this has become the accepted ruling.
The way it goes is this: Someone asks about if shield drones give the unit they are with a 4+ invuln save or do they only get an invuln save themselves with no additional effect to the unit they are with. It gets mentioned that they are wargear and the person asking figures that the unit they are taken with would get that 4+ invuln. The majority of the responses say that only the shield drone gets the invuln. This is wrong... and this is the reason why:
~ Shield Drones give a 4+ save to any unit they are attached to (taken as wargear or even if an HQ joins up with a drone squadron made up of these).
Pg. 55 of the Tau codex- WARGEAR: shield generator (Shield Drone Only)
Pg. 69 of Tau Codex- Shield Generator description: A shield generator confers a 4+ invulnerable save.
The key word there is on pg 69 where it says "confers". To confer means to give something to someone or something else. So this is not a reference to the drone itself, but saying that it "gives" a 4+ invuln save. Just because the words "to any unit it is attached to" are missing, doesn't mean that the intent isn't obviously clear. If this is still confusing to some or you don't want to take my word for it- I even looked up the meaning of the word confer in my "Merriam-Webster Dictionary" so I wouldn't just be talking smack.
Confer: 1: GRANT, BESTOW 2: to exchange views : CONSULT
I even did a Google search to see if the definition would be diferrent. Via Google search:
1: Grant or bestow (a title, degree, benefit, or right).
2: Have discussions; exchange opinions.
consult - bestow - grant - give - award - accord
The words there to focus in on are "give" and "award" in addition to the others...
So, in other words- the shield drone does not "bestow" the save unto itself, but to any unit it's attached to. Thanks all for reading and I hope this clears up any further confusion on this rule and hopefully they will amend the description in the next FAQ to make it more clear for others...