In The Raven episode of Star Trek: Voyager, Seven of Nine, who is disconnected from The Collective, is able to recall species numbers, dates, and other details of assimilation for Talaxians (Neelix) and Vulcans (Tuvok).
Does this mean that each Borg drone has an exact copy of the entire collective memory? Or are there any other explanations on how Seven of Nine could remember so many details despite never having taken part in these events, or events prior to her being assimilated?
It is physically impossible for any individual drone to store that much knowledge. Bear in mind that Seven's parents were sociologists who were studying the Borg Collective; it makes sense that her mind would be full of interesting tidbits about Borg history, even before her assimilation. Since Borg drones retain their memories of their lives as individuals, it makes sense that she would possess this knowledge. It is likely - though never outright stated - that individual drones have information necessary for their duties 'downloaded' into their brains before performing those duties; this is insurance against any communications issues which may leave the drone out of contact with the Collective.
There is evidence of this in Regeneration, where two drones who are out of contact with the Collective are still capable of building both a transmitter and a primitive ship just using their own knowledge.
No, that would be hugely inefficient and greatly limit the amount of knowledge the Borg collective can accumulate and process.
Borg drones more than likely receive a basic core data set then gradually assimilate information specific to their role as the need arises or as their role changes. Additionally, their cortical implant acts as nodes in a distributed file system for assimilated neural patterns, and their neural processor also records all information received from the collective, giving drones much better memory than the average humanoid.
Lastly, Seven of Nine probably knows more general historic and comprehensive operational/technical knowledge than most drones because she was the tertiary adjunct of Unimatrix 01, a subsection of the collective that worked closely with the Borg Queen.
I think its more like able to access a Database rather than memory stored in their heads. That's why Pickard severed the spinal column from the Queens head in First Contact, otherwise they could have plugged her in to an access port a steal all the information about technology and whatever the Borg poses.
And as for 7 of 9, it could be because she has parts of the DB stored that she needs to retain for certain things like recognizing species and some technical stuff. But its so much is like she is a computer.
Answering my own question. Today (early 21st century) I can have entire Wikipedia or (probably) a copy of entire Stack Exchange network stored on 1024 GB pendrive that is smaller then my nail. Provided that I strip it off all the "social data" (see below) and keep facts only. Or I can have entire human music collection on my just-a-little-bit-enhanced iPhone.
And both things are so obvious to most of us that we simply don't think about them this way.
It is not a "database" that formulates "big data". It is our (human) "social data" that makes it. Where we where, what we did, what we see, what we bought, what we think, what photos and movie we have. It has nothing to do with things like Wikipedia, Microsoft Encarta or The Great Congress Library.
When we consider Collective knowledge as just another huge database that stores only facts about ten million different species (random number; not a fact) they have ever encountered then I personally think that other answers given here are simply incorrect. It is not only possible, but also very efficient.
I can very easily imagine that after next 200-300 years it will be possible to store entire humankind "database" within single human hair. If we have at least 10k hairs then we can store entire Collective knowledge within each and every drone. Without even a tiny problems.
