Plugging in an external hard drive usually results in Spotlight indexing the contents – or just upgrading the index that already exists. This can be avoided by excluding the hard drive from being indexed under System Preferences – Spotlight – Privacy if desired.
But, most often than not, we actually need to use Finder to find file on our drives, so having Spotlight build and keep the index up-to-date is quintessential. It can surely be one of the culprits keeping our drives busy when the system is idle – however, not the only one!
To check what processes are accessing what files on our disks in realtime, we can use fs_usage and pipe it through grep to narrow down the results.
Open a Terminal and substitute Triforce with the name of your external drive (or any other mount point, really). It has to be done as root, so we are using sudo to elevate our rights – meaning that you will have to enter your password first.
sudo fs_usage |grep Triforce
This will keep churning out access data regarding all processes that touch your external drive in any way.
To stop / quit, press CTRL + C.