I currently run 3x 16TB Seagate ironwolf pro HDDs, in a raidZ1 so one parity drive. I recently got a great deal on two more which I have purchased and thankfully I haven’t yet used much of the space so my plan was to plug in an external HDD that’s 20TB (enough for all my data in the pool as of now) Make a temporary pool with it and use the mover to move my shares where the data is to the external. Then I remove the existing pool and make a new one with the extra two drives and use the mover again to get the data from the external drive to the new pool. Does this sound like the best/easiest option for this? I can also do it on windows on my main PC if that’s a better option. 

You can do that but the mover won't work, you can use for example the Dynamix File Manager plugin.

Ok I’ll have a look at that, thanks. Out of intrigue how come mover wouldn’t work?

Mover currently can only move from one pool to the array, or from the array to a pool, not from pool to pool.

 

 

 

 

Ok I’ve downloaded the file manager and will try it out, thank you 😄

Are they all in a zfs pool?

Tehn zfs terminal command to disk replce and partiy/replicate build.

https://docs.oracle.com/cd/E19253-01/819-5461/gazgd/index.html
https://docs.oracle.com/cd/E19253-01/819-5461/ghzvx/index.html

In quick recap you bring the pool offline.  
You unconfigure /remove the disk
add the new higger capacity disk
run a scrub/parity rebuild
^replace all disk with higher capacity drives to increase space...