Would like advice

I have the 5 Bay Drobo S with 3 2T and 2 4T. I run out of space about once every 4 weeks. I only know of 3 solutions. Buy 3 $t to replace the 2T. Buy a daisy chain set up. (I don’t even know if that is possible), or buy a whole new system. Would like an opinion of people in the know.

You dont have to buy three 4TB drives, just buy a single 4TB drive and remove one of your 2TB drives, that will immediately give you an additional 2TB of free space