Truenas 12 Can't Mirror - Error: "can only attach to mirrors and top-level disks"

The only thing to fix here would be more accurate error messages. Those are coming from upstream OpenZFS, so the issue could be reported there. We are very limited on time now, sorry.

I've reproduced this issue on my test system with two different devices.  Addition of `-o ashift=9` into the `zpool attach` command allowed the attachment to succeed, but resulted in `block size: 512B configured, 8192B native` message in `zpool status`.

Looking into the code I see that "can only attach to mirrors and top-level disks" message is in fact reported on many other reasons when disk can't be added to the pool (ENOTSUP).  In this particular case I bet the problem is in physical sector size of devices, reported as 4KB by the SSDs and not reported by the USB stick and probably defaulted to 512.  You can not add into mirror with existing vdev with ashift=9 vdev with ashift=12.  It should probably be possible to force ashift=9 for the added vdev, since 512e disks support smaller I/Os, but they will work suboptimal.  So instead of the new SSDs addition into the mirror I would recommend you to backup config and reinstall from scratch onto the new devices.

@clinderwell I added mine when I added my comment. @Bonnie Follweiler mine is available. Just let me know if you need anything else.

@Bonnie Follweiler - I can try this later. I ended up just making an entire SSD a boot drive and don't have a comparable SSD to run this again on. I can likely do this with the USB Drive I was trying to use before but need a few days. 

@John Moberly/@Keynaan - Any chance you have this debug that you can easily pull?