Yes BIG problem. Raid 1 is not a data backup. If the box blew up when you plugged in that drive how would you get your data?
Get an external 2tb usb drive and copy/backup your data to it.
As far as download the ISO, that is up to you. If the two 2tb drives are old/having problems and you do not want to use them, then yes.
But if you keep all 4 of the drives in the box you will have 6tb of storage. If you only use the two new 3tb drives, you will have 3tb of storage.
If you now try to use only the two new 3tb drives you will have to reseet everything which means zero out your drives. SO you must backup your data.
RAID is not a backup. If there is any thing on the DX you need or would like to keep, you need to make a copy of it.
