Mount directory into zfs tree

I have a zpool which is a 2-disk mirror. It is, unfortunately, running out of room. So, my plan was to copy off a large directory from the zpool and put it onto another hard drive until such time as I can upgrade the disks.

However, I have not been able to mount this external disk INTO the zpool.

My main zfs mount is /opt/data. I am trying to do a mount /dev/sda1 /opt/data/large_folder and it completes with no errors but is not mounted.

I also tried mount /dev/sda1 /opt/data2; mount --bind /opt/data2 /opt/data/large_folder and that does not work either.

Unfortunately symlinks wont work as this is for nextcloud and that software does not allow symlinks to be followed.

Any thoughts or advice would be greatly appreciated.

FYI you don’t need to replace the disks outright, you can add a 2-disk mirror to an existing mirror pool to make a “raid10” equivalent. See line 117 of example code:
Make sure you set “autoexpand=on” first

I had a similar scenario, my main pool was getting full so I moved a large directory into a secondary disk. However, I made a single disk zpool. The dataset from this secondary pool is mounted in the tree of the original pool in place of the original directory. My expectation though is that it shouldn’t have to be ZFS; that an ext4 file system or something should also be able to mount in the same tree. Just some thoughts as I can’t really think of a solution/advice, and hope you’ve had better luck recently.