adding 2 4TB arrays

I have tried a few things - but sadly I am not able to figure out how to mount two 4TB drive arrays ...

Any ideas?

I have thought about breaking them down to be smaller - but we would loose the ability to have a raid 60 array that way

Please advise.
 
as additional drives to an existing installation or as the installation drive?

installation drives are currently limited to 2 TB.