TIME Machine External Hard Disc, exclude permanently

I have regularly connected an external hard disc to my MacBook where I save my photos. I do not want to back up time machine this hard disc but do this separately. In the time machine menu I can exclude this external disc and the folders that are on it under "Options". Does time machine save this permanently or do I have to exclude the external disc / the folders whenever I switch on my machine? I am suspicious because after a restart I do not see the excluded folders under options in time machine.....
Is there a way, that I can exclude the back up of this external hard disc permanently? Thank you for your answer

External drives are automatically excluded from Time Machine backups by default unless you go into TM preferences and remove them from the exclusion list.

They are not going to appear on the list except when they are connected.

Thank you very much for your reply. But I still did not get it: Does it mean I do not have to bother? The external disc will not be backed up even if it is connected?

Yes, the external disc will not be backed up unless specified.