In my main machine right now are two 4TB hard drives, in addition to several SSD's. One of the two drives is used for photo storage, the other for all other large data needs (Dropbox, video projects, email, virtual machines, etc). However, neither is near full, heck they are both less than half full. Then the data on them is rarely used for the most part. For photos, I only really work with current ones and while I do need to go back and search for older images, these could easily be on the network. The reduced speed of images placed on the network is not a real problem for these older files.
My file server is a homemade computer running and OS called unRAID, but any solid NAS would provide most of the same options discussed here. My file server currently has 32TB of storage and can be easily grown to as large as 80TB or more. So storage space is not an issue. My server also runs Crashplan, backing up all the files to the cloud all the time. It also backs key files up to some external drives for added local protection.
Given the strenght of my file server, I am making the leap to remove all hard drives in my main PC (eventually all machines in the house except the server) and go 100% SSD. Already my OS drive, my program drive and a cache drive for Adobe run on separate SSD's. But now I will be moving all my data onto an SSD. Speed is the main advantage here, but it also helps a little with heat and some other minor benefits.
Here is the SSD I am looking to buy, a 1TB drive for just over $300. Still a lot of money, but prices keep getting better on SSD's.