No way that I know of. The usb drive wasn't mounted on the system, it was added to the pool as another drive. Once that happens, it becomes a required item for operation, even though there may not have been significant data on it. It is also an irreversible action. Once it was added, it can only be removed by being replaced by another device that takes its place. In the future, a method of retrieving the ZFS data from the original drive may be available.