Disable indexing of ALL attached external drives. Replace PATH_TO_NOINDEX_SCRIPT with its name's-sake in the .plist file.

com.index.no_index.plist

<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE plist PUBLIC "-//Apple/DTD PLIST 1.0//EN" "http://www.apple.com/DTDs/PropertyList-1.0.dtd">
<plist version="1.0">
    <dict>
        <key>Label</key>
        <string>com.index.no_index</string>
        <key>Disabled</key>
        <false/>
        <key>Program</key>
        <string>PATH_TO_NOINDEX_SCRIPT</string>
        <key>StartOnMount</key>
        <true/>
        <key>RunAtLoad</key>
        <false/>
        <key>KeepAlive</key>
        <false/>
        <key>EnableTransactions</key>
        <true/>
    </dict>
</plist>

no_index.sh

#!/bin/bash

for filename in /Volumes/*; do
    if [ ! -L "$filename" ]; then
        file="${filename}/.metadata_never_index"
        if [ ! -e "$file" ]; then
            touch "$file"
        fi
    fi
done;