Slower drives generate less heat. There is no air-flow inside the DMS casing. Some 7200 rpm drives can get quite warm. Most 5400 rpm units will be OK, but try to look up manfacturer's specs if you can and compare. Normal "idle mode with the drive motor running" power dissipation figures are a good guide, since the drive does not get that busy. I don't think Phatbox users have had any major issues with 5400 rpm drives though.