O'k how about we go with this. The logs for Enterprise and their encounter with Borg were in the Ent D computers but Picard and co were taken to the Delta Quadrant (or close it) and he just didnt think to ask the computer if it had any record of said enemy. When Guinan said they were the Borg, he asked the computer if there was any reference to them and as they didnt announce themselves in Ent, the computer didnt know what he meant. Maybe Picard needed to ask the rigt question. Of course this still doesnt explain why there wouldnt be a record of "assimilation" in the medical records AND the knowledge that Denobulins (or was it just phlox) were resistant.Lets just say all the records were lost in the "fire" at Star Fleet. And that is a whole different story. Either way, by the end of Regeneration, the Borg were a pale comparison to the scary things they were in Q who. Borg, i might add that woulld have come AFTER the scary Q who borg.