I stopped my array, changed the number of array devices to 12 and then changed cache devices to 4.
The admin page is stuck at "Mounting disks..."
I see in the syslog:
Jan 11 07:58:45 arwen emhttp: shcmd (169): mkdir -p /mnt/cache
Jan 11 07:58:45 arwen emhttp: mount error: No file system (no btrfs UUID)
Jan 11 07:58:45 arwen emhttp: shcmd (170): rmdir /mnt/cache
Jan 11 07:58:45 arwen emhttp: shcmd (171): sync
Jan 11 07:58:45 arwen emhttp: shcmd (172): mkdir /mnt/user0
Jan 11 07:58:45 arwen emhttp: shcmd (173): /usr/local/sbin/shfs /mnt/user0 -disks 1022 -o noatime,big_writes,allow_other |& logger
Jan 11 07:58:45 arwen emhttp: shcmd (174): mkdir /mnt/user
Jan 11 07:58:45 arwen emhttp: shcmd (175): /usr/local/sbin/shfs /mnt/user -disks 1023 2048000000 -o noatime,big_writes,allow_other -o remember=0 |& logger
Jan 11 07:58:45 arwen emhttp: shcmd (176): cat - > /boot/config/plugins/dynamix/mover.cron <<< "# Generated mover schedule:#01230 0 * * * /usr/local/sbin/mover |& logger#012"
Jan 11 07:58:45 arwen emhttp: shcmd (177): /usr/local/sbin/update_cron &> /dev/null
So what now?