Creating a Dataset via the "ZFS Datasets" page doesn't work the first time. To successfully create a dataset, I must:
1- Create a dataset, then "apply changes"
2- Delete the dataset, then "apply changes"
3- Create the same dataset, then "apply changes"
After step 1, the "Dataset Information" page says:
"cannot open 'tank/test': dataset does not exist"
After step 3 I can hear my disks reacting and actually creating the dataset.