At a prior employer, we had racks of Dell servers, each with their own disk boxes attached to RAID controllers. Sometimes, more than one of the servers at the

At a prior employer, we had racks of Dell servers, each with their own disk boxes attached to RAID controllers. Sometimes, more than one of the servers at the same time would decide that their respective disk enclosures had disappeared and reappeared, and the RAID controllers would be unhappy and mark the volume as foreign until a human intervened. Windows and Linux both did this, so it wasn't an OS problem, and it was multiple machines in multiple racks, ostensibly UPSed with line filtering. The odder thing was when we noticed it was almost always the ones on the upper half of the racks. The best, though, was what "resolved" the issue - the power exchange next to the building blew up one day (AIUI one of the phase lines for the three phase connected to another phase's busbar, and BOOM, the room was covered in a fine copper mist), and after all the power equipment in the exchange was (eventually) replaced, the problem went away.