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Originally posted by Capt.Hunter
Well, "Scorpion, Part II" did say that it was originally the Borg who invaded fluidic space, rather than Species 8472 invading ours. . . .
When Q first introduced the Enterprise-D to the Borg, the Borg initially beamed over and began assimilating data and technology. . . which means they probably gained access to mission logs of the original Starship Enterprise. . . including their jaunt into the Mirror Universe. So, if they weren't aware of the Mirror Universe at that point, they certainly knew about it after their encounter with Picard's Enterprise.