I recall the day I went into the computer shop complaining about the lack of space on a DD disk and I was sold the XL drive and never really used anything else. They look exactly the same as a normal floppy and I have managed to get a couple thinking they were just your normal drive. They are a bit slow I have to say.
I think this is what you are talking about.. Just works as a HD drive.
There is an internal version as well, which is what I have. It looks to me like the extra circuit board that attached to the back of the (Sony made) PC HD floppy drive could be attached to any HD floppy drive (I have tested it with other PC drives in DD mode and it did work. I never had the opportunity to test it with other PC drives in HD mode).