I got a new server for Dandandin and I kept it open from all sides while I was doing tests. Average disk temperature 23 ° C. When I saw that was stable, I closed, placing fans on the front and on the back, to have a positive airflow. Disk temperature lowered a couple of degrees, and the disk has much more activity than last week!
update: looks like smartctl is not reporting accurately the hdd temperature. I need to do more tests with hddtemp.