trouble setting up raid1

[Bill]

Hi,

I'm trying to install a twelfth raid 1 partition on Jessie 8.5 and a 5 
TB HDD using gpt. Raid partitions 0 through 10 have presented no 
problem, but the twelfth pair of partitions won't pair up.

After I've selected /dev/sda12 and /dev/sdb12, Continue and then Finish 
from the menu, I've still only got Raid device numbers 0-10. /dev/sda12 
and /dev/sdb12 remain unlocked and available for reconfiguration - 
unlike devices 0-10 which show up on the main menu as Raid devices.

I'd guess that it's highly unlikely that there's a maximum number of 
Raid partitions, although I could live with it, but why is Partman 
rejecting my overtures?

And BTW is there any way to unlock or back out of a raid setup during 
configuration just in case I need to change something. How can I delete 
a raid device and start over if I have to? fdisk?

        Bill