One issue with running many drives is that it can overtax your power supply. At idle, a drive may only draw about 8 or 10 watts but on spinup the current drawn can be up 2.5x the rate when at operating speed. 4 drives can be a problem even with a 500 W power supply. An Antec Neopower 480 I have will not drive 4 Seagate Barracudas for example (on PC). There is a case therefore for keeping the drive count down. Two x 250 isn't necessarily a better idea than 1 x 500 or 1x 1000.