Replacing a failing drive


version: DS1821+
HDD Drives: (4) 12 TB Seagate Ironwolf Pro drives assigned as 1 volume
SSD Drives: (2) Synology 400GB SNV3410 as read write cache for the 1 volume

Data Scrubbing and extended S.M.A.R.T are performed every 3 months and last completed 8/16/2023.

This morning I was greeted with an email from my NAS that a UNC error was detected on Drive 3.

As the email suggested I started and Extended S.M.A.R.T that is currently 20% complete.

I am in the processes of ordering a couple new drives, 1 as a replacement for drive 3 and 1 to be a hot spare. Do I have to stick with 12 TB drives or can I get 2 16 TB drives?

I plan to eventually go with 16 TB drives for all slots.

Is there anything I am missing?