Data grid 426 was a memory address aboard a Borg cube disabled by the Brunali Borg virus in 2376.
This address contained a message from the Borg Collective after they received a message from the disabled ship. Although the Collective received the message, they chose against sending aid as they did not tolerate imperfection and the survivors of the virus were considered irrelevant.
The survivors had access to this data grid after using decryption protocol theta three. (VOY: "Collective")