Tips for shell based partitioning during install

[Peter Pauly]

I'm using the option in the installer where you go out to a shell prompt
during the partitioning step in the installer on FreeBSD 11-Release and
booted off of the CD. All is going well until I get to this step:

zpool create zroot raidz2 /dev/gpt/disk0 /dev/gpt/disk1 /dev/gpt/disk3 ...
etc.
cannot mount '/zroot': failed to create mountpoint

The instructions when going out to the shell say I have to mount the
filesystem under /mnt but /mnt is read-only.

I tried to use the Guided Auto (ZFS) but it doesn't work with more than 10
drives.

What am I doing wrong?