External Drive Constantly Indexes.

I have a couple of external hard drives attached to my Mac, but one of them constantly indexes, usually not stating for how long, when I drag it into the Spotlight/Privacy window to stop it then remove it to allow indexing it resumes usually stating ho long is remaining.

My question is why does Spotlight randomly index this external hard drive.

Thanks....As always.

 

Thanks for the replies.
I use it mainly to download NZB files onto, before I transfer them to my Popcorn Hour.
So it gets used a good bit every day.
What if I exclude just the download folder from the drive in question, as shown below.....Might that help.

 

Thanks for the replies, but I still want to use Spotlight to search....Just not to index it all the time.