Some commodity machines use external SAS to connect to more disk boxes. IMHO, there's not a real reason to keep images and tweets on the same server if you're going to need an external disk box anyway. Rather than getting a 4u server with a lot of disks and a 4u additional disk box, you may as well get 4u servers with a lot of disks each, use one for tweets and the other for images. Anyway, images are fairly easy to scale horizontally, there's not much simplicity gained by having them all in one host, like there is for tweets.