My set-up has three internal drives...one 750GB/10,000 rpm drive and two 2TB/7200 rpm drives. I have space to add a fourth drive, but haven't. None are SSD. Currently the applications are on the fast drive (LR4 and CS5) with one of the slower drives containing my User Directory, LR Catalog, Pictures and ACR Cache. The second slower drive backs up the first via Time Machine. My basic question is: Would I gain anything by putting the ACR Cache on the faster application drive? Would program execution suffer? Any other comments on the LR set-up definitely welcome.