Ah! that would explain things in a different perspective: I seem to have had less issues since I decided to test the NTFS formatted HDD, I was getting all sorts of weird things with exfat. I was beginning to suspect a possible trigger being "not stopping a media file playing" and just letting it finish (in my case music). I was beginning to feel that stopping playback and / or going back to the home screen seemed to make the problem of weirdness somewhat less. A bit like a file being left open, and the O/S / hardware times out, disk spins down or such-like and when it wakes up in response to a key-press it gets in a state because it assumes that because the file is open, the hardware must all be awake and the HDD spinning, a (e.g) read command is issued and the system gets in a state because the HDD is not spinning, and expects it to be so, therefore "sulks" rather than waits for a spin-up causing the disk to disappear from a DF list...
Just random thoughts from what I observed when using exfat. Because NTFS has been around for longer maybe those sorts of bugs have been ironed out in fuse.
So in a perverse sort of way, I am saying that NTFS seems to be more stable than exfat using fuse.