Performance on a clustered SQL 2005 instance

Customer sez: I've been getting timeouts all morning
Kevin3NF sez: CPUs are good, memory is fine (buffer cache hit ratio and page life expectancy...my two favorite memory counters for quick hit analysis..)
Kevin3NF: Uh-oh....why is the disk queue so high for drive N? Houston, we found the problem...need clearance for root cause.

Root cause: Someone decided to expand the volume on the SAN that included the data drive, T-log drive and backup drive while the instance was running, and at HIGH priority. The second it finished, everything went back to normal.

Sadly, I needed a Windows/Storage guy to tell me what was going on after I identified I/O as the resource bottleneck.