Disable Spotlight for external drive

Use mdutil command to prevent indexing on external drive.

First check status for all:

mdutil -a -s

Disable spotlight for all and enable your home drive and /System/Volumes/Data (all apps) explicit:

sudo mdutil -a -i off

sudo mdutil -i on /Volumes/OSX

sudo mdutil -i on /System/Volumes/Data

Or check if the external drive has indexing enabled or not:

mdutil -s /Volumes/DRIVE

To disable indexing, run the command:

sudo mdutil -i off /Volumes/DRIVE

You can recheck the indexing status with the first command again:

mdutil -s /Volumes/DRIVE