- `System Preferences.. > Spotlight > Privacy`
- Add the mount path you don't want indexed to the list by selecting the device you want excluded also. However, if it's hidden use <kbd>**⌘**</kbd> +<kbd>**⇧**</kbd>+<kbd>**G**</kbd> and enter the path name for example `/Volumes/Partition Y`.

**Note**  
After some testing this method does not work with multiple partitions of like OSs (eg. 10.8 on partition A and 10.8 on partition B) but does seem to work if each OS is different. Still not 100% clear on why this is since each drive has it's own `/.Spotlight-V100/VolumeConfiguration.plist` or `/.Spotlight-V100/Store-V1/Exclusions.plist`