I have a zfs raidz-2 (raid-6) with 7 4TB hard drives. When I added another hard disk with
zpool add, it was not included in the original raid (zpool status). Can this still be fixed? Or do I have to create a backup and recreate the pool?
Thank you for your help.
This can't be fixed, that's the way ZFS works. This is a classic newbie mistake: you can't extend a vdev. Worse, if the newly added drive fails, it will fail the whole zpool, including the perfectly sane RAID-Z2 vdev, and you'll lose all of the zpool data!
For a complete list of common ZFS pitfalls, see this very detailed blog post.