Is there a way to limit external hard-drive random spin-up?
This problem has been happening since January 2010, according to users on this Apple Discussion forum. While no fix has been identified, two potential culprits were identified:


Spotlight - one user (loayza) noted:



  I also have the issue (SL 10.6.8). I have an external drive to handle
  TimeMachine and every time I select "Open with" from the contextual
  menu it wakes it up. It's quite annoying! It seems disabling Spotlight
  solves the problem...



iStatPro - one user (jrawl004) noted:



  I found the culprit.  iStatPro.....I have a habit of tracking my
  system status while editing video and this program does a great job at
  keeping things up and reportig.  So everytime I made the slightest
  touch to my iMac or MacMini, it would awake everything so it can
  report.  I disabled the widget and low and behold, things became
  extremely quiet quickly.  That problem is solved for now....for me at
  least.


No solution was identified when checking Apple Support's KB.