Sorry to revive this old thread, but I experiencing the same issue, apart that the "volume status" never show up at the bottom. I even upgraded FreeNAS to 9.10 in the hope it would show up but it does not.
I know I could probably do this on the command line, but by searching intensively on the forum for an answer to this, I was led to believe that the GUI do not sync itself to the system, but rather the system is sync to the GUI/DB... in a nutshell, don't do it from the shell.
So, how can I detach a disk from a zpool through the GUI???
In the screencap, I would expect by selecting my volume to see a drop down of the disks attached to it.
Thank you.
PS. I'm attempting to offline/detach disks from a raidz1 pool and then attach/online/resilver bigger disks 1-by-1 and then resize the available space to 100%.