Grant: 2TB drives are pretty cheap thes days. I just replaced two of them (one was soft-failing) with 4TB units, for a total cost of around £175. The 3-disk array of which they were part already had a 4TB drive to which replaced an earlier dodgy unit. Once all three were 4TB the array could be grown to provide an effective 10.5TB filesystem.
The extended array (actually a ZFS pool) is already 22% full because it holds all the archives & backup of all the systems on the home network.
Another 4TB unit is inside a portable USB-3 drive. Very useful for ferrying material between the UK and LP.