Also, a device that connects via USB to supply storage is not and can not be a NAS. NAS = Network Attached Storage.
What is described here appears to be a DAS, Direct Attached Storage.
USB attached storage with ZFS can be problematic. Not saying it will happen in this case… But this forum and the old forum are littered with sad stories of people having trouble due to USB attached storage. Some even with data loss. Below is what I wrote on the subject, but again not all will apply to all cases:
