While testing various memory disk filesystems in FreeBSD, I intelligently made changes to /etc/fstab that pointed to non-existent mountpoints (zroot/usr/home did not exist until later in the boot process - and the system refused to boot into FreeBSD due to mounting errors.)
To remedy this, I booted into my Ubuntu Oneiric installation, which fortunately supported native ZFS (follow this guide). Using the following commands, I was able to mount my root partition zpool - zroot - to make the necessary adjustments to /etc/fstab.
In Ubuntu, as root, list the available zpool imports with:# zpool import
This will reveal all existing zpools found on all disk drives. Check to see if the zpool can be properly imported by reading the state, status, and action descriptions of the zpool.
Continuing on, create a temporary mountpoint for the zpool:# mkdir -p /mnt/zpool_temp
Then import the zpool:# zpool import -N -R /mnt/zpool_temp zroot
To confirm this worked, do:# zfs list
Also, note the dataset’s original mountpoint value with:# zfs get mountpoint zroot
Then:# zfs set mountpoint=/mnt/zpool_temp zroot# zfs mount zroot
This will temporarily mount the root zpool R/W at /mnt/zpool_temp, allowing you to make the necessary changes.
After you are done making changes to the mounted dataset, while making sure that it is no longer in use:# zfs unmount zroot
Then, reset the mountpoint to it’s original value:# zfs set mountpoint=/ zroot
Now reboot the machine without exporting the zpool - this is very important.
Your FreeBSD should now boot properly.