Teardown update: We tossed the One's hard drive into one of the tech writers' secondary hard drive bays. We found five NTFS partitions:

Temp Content: 44 GB capacity, 27.1GB available (in an Xbox that we never turned on).

User Content: 391.9 GB capacity, none used.

System Support: 42.9 GB capacity, 34.1 GB available.

System Update: 12.8 GB capacity, 11.8 GB available.

System Update 2: 7.52 GB capacity, 7.47 GB available.

Sadly, we were not able to go the other way -- we didn't have a spare hard drive (in New Zealand) to put into the Xbox One. So that will have to wait for another day!