Dust is mainly killing drives by
- attaching to the drives so they get isolated and warm.
- attaching to the fan and other places so the airflow is hindered and the drives get warm.
Dust does not get into a normal drive, just like Docchris said.
The easiest and cheapest way of cooling the drives, keeping dust out and make it more silent is to replace the front. Just place a 140mm fan in front of the Drobo like I did here:
You should buy a filter paper or just cut some paper from a bag made for vacuum cleaners. Place the paper either in front of the 140mm fan or between the fan and the Drobo. You could vacuum your filter when it become dirty. Replace the filter when it doesn’t look clean enough.
Do not add a filter without adding a fan. The Drobo got poor enough airflow as it is.
A drive should feel pleasantly warm to touch when it got its ideal temperature, 30C to 45C. It is definitely too hot when it is uncomfortable to keep holding your fingers at it, 60C.