Mount & unmount volume notifications
If Growl is installed and you want to turn off just the notifications for volume mounts and dismounts (as opposed to getting rid of Growl altogether):


System Preferences, Growl panel
Applications tab
Select HardwareGrowler from the list
Configure
Notifications tab
In the dropdown, for each notification (e.g. Volume Mounted) you want to disable:
Select it and untick the 'Enabled' box.


(Edit:) Or, as fnurl points out, remove HG entirely.
I find it useful enough to keep, but tweaked to my liking.  It alerts me if my WiFi starts bouncing, f/ex.  YMMV, as always.