Every once in awhile when you attempt to remove a dataset in FreeNas you may run into the error “cannot unmount dataset: device is busy”.
This usually means that a user has a share mounted or a service is running that is currently using that dataset. However, if you have confirmed that this is not the case, you force an unmount of the dataset using the following shell command:
sudo zfs unmount -f /mnt/VOLUME/DATASET
Simply replace VOLUME and DATASET with the proper values from your environment.
Once that processes, you should be able to remove the dataset normally from the GUI.