I added the GEOM Gate device to the #ZFS mirror because I wanted to switch disks. On the screenshot, ada1 only had 128GiB and the disk behind ggate0 had 1TiB and was the one I wanted to use in the future. I prepared ada1 with “zpool set autoexpand=on mypool” and “zpool online -e mypool ada1”. Then I added the 1 TiB disk to create the mirror. When ggate0 was done with the resilvering, I detached ada1, powered the two machines involved off and physically switched disks. I’m wondering if that works just as good when the machine in question is still running.