I once had two 5Tb drives die on me within weeks of each other. I can't even fathom putting 12Tb on one drive without a double backup.
If my main external go down, I do not use the back up external at all. I wait until I’m able to afford another one, then I transfer my data to the new external and use the back up as the main one. The new external becomes my back up drive, which is stored away.