Truenas forum. Level 1 Techs youtube channel. Lawrence Systems youtube channel. the man pages of course. First thing I'd suggest is really getting the termino

Truenas forum. Level 1 Techs youtube channel. Lawrence Systems youtube channel. the man pages of course. First thing I'd suggest is really getting the terminology under your belt. Everything makes so much more sense when you use the correct terminology. For example, you don't extend a drive. That doesn't really make sense. I suspect what you mean is that you added a vdev to a zpool. Don't think about things in terms of disks and extending them, think about things in terms of one or more disks making up a vdev, and a zpool is, well, a pool of those vdevs. Zfs then works out how best to write data across the vdevs in the pool. Not disks.