Using Time Machine to back up an EXTERNAL drive

My understanding is that Time Machine automatically excludes external hard drives in a backup. I keep my Photos library on an external drive and would like to have a backup of my photos. I have gone in to the Options for Time Machine and that external drive is not listed as excluded. I have to assume that it is included, although that goes against all the Time Machine rules I've seen. Is there a circumstance under which an external drive would not be excluded ?