External hard drive keeps removing itself from Spotlight privacy list

Hello,

My system is a MacBook Air (13-inch, Mid 2012), running Sierra v. 10.12.1

I have a nagging problem with not being able to prevent Spotlight from indexing an external hard drive. The problem arose as I noticed I often wasn't able to unmount the drive because of a Spotlight process indexing it (no other program was using the drive). So I decided to add the external hard drive to the Spotlight privacy list. The trouble is that this setting does not stick: if I unplug the disk and plug it back, it does not show up in the privacy list anymore, so I have to manually add it again. But it actually gets worse: sometimes, after I have plugged in the drive and added it to the privacy list, it mysteriously disappears from the list (and that's when the drive cannot be unmounted anymore -- I have to force the unmount).

I have scoured the internet for solutions and tried them all (e.g., adding a .metadata_never_index file to the root directory of the external drive): the relief only seems temporary and the issues rears its ugly head again after a short time.

Has anybody here faced a similar issue and come across to an effective solution?

thanks in advance for any comments!

best

giuseppe