meant that you have to be in single user mode until you have edited
/etc/fstab to point to the mirror, otherwise you wouldn't boot with
root on the mirror. The synchronization between the disks works fine
in multi-user mode as well.
I have two 2 TiB disks in gmirror set up just like that.
Synchronization was done running in multi-user.
You are right - just removed and then re-inserted a component in one of
the mirrors and the mirror synchronized fine in multi-user mode.