When you add a new disk device to the “zroot” pool (or whatever it is that you’ve named the ZFS pool where your root partition resides) you should also add bootstrap code to that specific disk, so that the system can actually boot from it should the other disk(s) in the pool suffer a hardware failure.
Assuming that you’re using disks partitioned using “gpart” and have an EFI partition, your disk might look something like this (using “gpart show da3”, for example):-
=> 34 7814037101 da3 GPT (3.6T) 34 6 - free - (3.0K) 40 1024000 1 efi (500M) 1024040 12582912 2 freebsd-swap (6.0G) 13606952 209715200 3 freebsd-zfs (100G) 223322152 7590714976 4 freebsd-zfs (3.5T) 7814037128 7 - free - (3.5K)
You also use “gpart” to write the boostrap code to your new disk. In this example, the command would be:-
gpart bootcode -b /boot/pmbr -p /boot/gptzfsboot -i 1 da3 partcode written to da3p1 bootcode written to da3
Note that “gpart” confirms that it has written to the disk.
PLEASE MAKE SURE that the you change the disk device specifier (“da3” above) to specify -your- correct target disk device. This command will quite happily destroy filesystems if you get it wrong.