The Problem is that drive partition is mounted by unassigned devices (is the automatic Setting for known filesystems) before "targetcli" can load it as a "block"-device. So targetcli exited with "Drive is Busy..."
So maybe it would be better to passthrough error-message of targetcli to WebUi or check if the marked device is mounted and unmount "before" targetcli is called, only thoughts to make the plugin more comfortable, because no message of targetcli is piped to WebUi.