Well to tell you the true.. there is no such thing max speed for racks
that because it depend on the HD speed, the only thing that matter is the ribbon that connect the rack to "base rack" but it such support at ATA 66. Only certain is if the plastic can dislate the heat. but a fan in the right location can solve this problem. best not to use 7200 rpm in there. If it must just make sure it not in use most of the time... (eg use 4 store only = is okay)