cheap drone flies conductive dust to the air conditioning systems? Like powdered metal dumped somewhere very sensitive, where it can be carried to sensitive equipment by the air pumps? Or fire/smoke at the intake?

Wouldn’t work. Dust is a major problems for data centers, and they already have pretty strict air-handling controls to prevent it.

Your main ways of disrupting one is by taking out the power, the HVAC, or the structure. I think out of the 3, the HVAC would be the easiest to disable from the outside. That being said, datacenters are not inherently problematic, so make sure you know what you are trying to destroy before you actually destroy it.