Ohhhh wait it just came to me. I did set up a backup drive at one point. It was not part of the volume per say as it did not host any data. But I'm guessing that's the missing device it keeps asking for.
Yea thanks for the heads up again. That's the original plan, backing up data to an external at the moment. Purchased 2 more drives and will deploy ZFS2 afterwards.
RAIDZ2 will be a much more stable and fault-tolerant arrangement than the RAIDZ1 you thought you had, and vastly more so than the RAIDZ1 striped with a single disk that you actually have.