As you’re pretty aware of, HTC include a new File Manager app in the 4.4.3 update for M8 (M7 as well but it’s less revolutionary on that phone).
That problem was: in 4.4 there was a security “measure” introduced that didn’t let just anything create a folder on external storage. More on that HERE.
But! With the 4.4.3 update for One M8 HTC included a File Manager app that can take ownership of the external storage, and that fixes things. Now you can create folders on external storage and copy things between internal and external. Problem solved. Done.
It’s not the most amazing file manager app ever created but it does it’s things very well. Like copying things from internal storage to external.
Btw, the Sense 6 gallery can move to external storage as well. In case you’ve missed that.
The gallery shows a folder being created on the external storage (extsd or whatever you want to call it). With just the HTC File Manager app, nothing extra. No fixes that requires root.
You can freely copy between folders on external storage and internal. Just know that some folders are large and take a long time to copy. But, it works just fine.