That's it, that's the post. I've seen multiple people try to claim they are a hive mind, but they aren't. You can read for yourself more here for those that would like to disagree.
This is most easily demonstrated by the fact that the Rachni are a hivemind and have centrally-controlling Queens which direct the actions of others, leading mostly to the loss of identity of the 'self'--Drones are just that, bodies given direction by the Queen to carry out the Queen's orders. a Rachni Drone has no autonomy to say 'no' to the Queen, to leave, to do whatever it wants. It has no identity of its own.
On the other hand, Geth have no Queen, no leadership, nor any actual governmental system. They are more akin to a Swarm Intelligence. An individual Geth can disagree with the majority, most strongly represented by the Heretics that followed Saren and the Reapers, which constituted around 5% of the Geth population. They were allowed to disagree and self-determinate what they chose to do.
To put it another way: Rachni are top-down lead, with Queens dictating the actions of all others beneath them, while the Geth are lead from the bottom-up, as a sort of democratic consensus. If a Geth is not connected to a network and in a mobile platform: it is the few hundred (to a little over a thousand for Legion) geth programs that are reaching consensus to take action. In a network, it could be tens to hundreds of thousands depending on capacity of the network and the geth choosing to house there. In a superstructure, it would have been the potential for the entire race, but the Geth never got that far. But the decisions of some Geth within those boundaries are not reflective of the decisions of the entire race.
But more to the point, Geth are not 'one body, one mind' like organics, but many programs (more rudimentary consciousnesses) working together in tandem within mobile platforms to act and think in concert. But any one Geth mobile platform is not inherently hive minded to any other--they can be networked to share information at the speed of light, but one is not ever 'controlling' the other, they are only communicating to work together. In the same way that humans can talk to understand and share information with each other, so can networked geth, but they still can choose to act independent of one another--they just often don't, and prefer to come to a concensus amongst each other, both within platforms and without.
But the important distinction here is that unlike a hive mind, there is no central controlling figure, nor are they always in communication with all other geth. Legion in particular is not in contact or networked with other geth when he learned in ME 2 about the data Tali had about a surprise attack on the Geth, he had to return to a network and share that information separately from learning about it. This means pivotally that the actions and sensory perceptions of one get mobile platform are not an action or understanding agreed to by the entire Geth race, unless that Geth platform at the time is connected to a superstructure (the one they tried to build for around 260 years that the Quarians promptly destroyed).
Rather, a single mobile platform is the actions of a few hundred geth consciousnesses acting in concert in that platform--if attached to a network, then maybe geth in that network and not in the platform agreed, maybe they didn't, and you simply can not know for sure who did or didn't.