Intriguing development over the weekend.
I did some testing, with the advice you guys gave me above, and it turns out that it was SMART that was preventing spin-down.
I was a bit surprised - I was convinced that it was Emby that was causing all the issues.
This was a bit of an issue as I was then wondering how I can keep an eye on my disk health if I cannot use SMART.
However, I have just come across this thread:
[SOLVED] Disks do not spin down automatically
It seems that if the SMART interval is less than the idle spin-down time, the disk will never go to sleep.
Hopefully, this shall sort out my issue.