There is the possibility 12TB has the 7010 outdone.  You'll almost never believe it.  Original 7010 spec for HDD was 1TB max and 128GB SSD.  Well, in that case, I also have old Dell specs outdone.  But that was what was tested with the 7010 at the time. So, if BIOS dropping a 12TB drive ever becomes too inconvenient, one option is try using two 6TB 2.5" drives in the same space as one 3.5" HDD. I'm still glad you tried replacing the CMOS battery.  When there's the one time we don't think of the battery, then it's the [blankety blank] battery.  : )>
Strange issue. Bought a new hard drive for my Optiplex 7010 running BIOS ver A25. HD was not showing up in BIOS, so I checked to see and found the A29 update. Applied it and my new HD now appeared in the BIOS and in Windows and works great. Rebooting no problem, HD still shows in BIOS and in Windows. If I turn off the machine and power it back on however the HD no longer shows in the BIOS or in Windows. I found that re-applying the BIOS update fixes the issue for as long as I have the machine powered on. Even after powering back on, the BIOS shows A29 so the update is clearly in place, but somehow when I power off my machine, whatever fix the A29 update has on my system to allow it to see my new HD is gone. Thanks for any ideas!
Optiplex 7010 - New hard drive shows in BIOS after A29 update, but must 
re-apply update after powering on machine.