I was on FreeNAS-11.2-U2.1 and experienced and issue where I could not delete an extent from the iscsi section.
This turned out to be a bug in the new interface on this version. I updated to the latest and it still does not work.
What I had to do was use the legacy interface. The bug was not related to that interface. To get to it:
- Login to FreeNAS and click the gear in the upper right corner
- Click Legacy Web Interface