You can also create files on disk and add them to pools. There's actually a trick you can use to create a failed ZFS array (e.g. if you want to create a 5+1 ar

You can also create files on disk and add them to pools. There's actually a trick you can use to create a failed ZFS array (e.g. if you want to create a 5+1 array with only five disks), where you use dd to create a sparse file of the appropriate size, which lets you create a '1 TB' file while only writing 1 byte to disk. Add it to your zpool along with the rest of your disks, then fail it out, remove it, and replace it with a physical disk. It's a good way of taking a drive with 2 TB of data on it and adding it to a ZFS filesystem which includes that disk while also keeping the data, but without using a separate disk (or copying the data twice).