I have seen many posts that Windows 10 erroneously detects an external HDD as a SDD. My question is whether it is OK to use Defraggler on a HDD that is recognized as a SDD, which requires bypassing the message that warns against degframenting a SDD?
If you are sure that it's a HDD then it will be fine to defrag it.
(TBH defragging shouldn't harm a modern SSD either, but it won't realy do anything other than use up some of its write cycles unecessarily. Modern SSDs have many more write cycles that the early ones did).