OpenZFS on OS X

Are you sure that /dev/disk1 is the correct drive you want to add to your pool?
I am asking because on my M1 Macbook, disk1 is the internal pre-boot partition which you probably don't want to use for ZFS ...

On which operating system and Mac hardware (Intel or Apple Silicon) are you?

I'm fairly certain, yes. I believe mine is set up where disk3 is running the OS. It's not present in my zpool.

I have an early 2008 Mac Pro (Intel) running Yosemite.

I came across this post due to its high search engine ranking and I tried the commands provided by lundman, and they didn't work at all. I even upgraded to the latest O3X version (2.1.6 at the time of writing).

I couldn't seem to get anything to work and to be honest, I wasn't too intrepid about the matter due to the consequences of a borked filesystem.

So I'm writing this for anyone who may come across this in future.

I'm not sure how common it is to expand O3X pools, but I imagine that due to the vanishingly small amount of systems that could even support this configuration, it is not good. See this thread https://openzfsonosx.org/forum/viewtopic.php?f=26&t=3353&p=11545&hilit=expand+pool#p11545

In any case, I loaded up the latest FreeBSD image on a USB drive, rebooted into the shell and then expanded the pools from the Live Shell.

Your command should look something like this to format the USB drive:

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dd if=FreeBSD-13.2-RELEASE-amd64-memstick.img of=/dev/disk5 bs=1m conv=sync

When you're in the live shell, get the drive IDs using

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zpool status tank

Then expand them using the typical commands, like:

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zpool online -e tank gptid/00000000-0000-0000-0000-000000000000

In my experience of one test, I wasn't able to immediately verify that the expansion had taken place, so I exported and then re-imported the pool.