It is funny - because we have no clue what you have installed and running on your iPad - and you have disabled the only sevice the iPad does not know about. Well, obviously a service on the iPad leading to a spin-up ... It this a NAS or an iPad problem now? Obvious, a) the disk spun down as per the specs sheet before, and b) it spun-up as it' was obviously required to do so - aparently triggered by the iPad WLAN connection. Now, the fellow community members here have no crystal ball available.
Suggestions? Ask Apple, as it's their product, what is creating network connections
or use the standard procedure made available by QNAP to investigate. Or consider wait some minutes - does the disk stay up (so it's really related to an initial connection to the WLAN with the possible state change of some IOS processes, and (direct or indirect) to the NAS, or will the HDD spin-down following something more than the configued idle time? Alternate approach: Install a solid state disk if this spinup is deathly to the climate, the electric power bill, or the noise is simply to much.
Should you think I'm nasty? Yes, I am when it comes to these kind of questions. There are no "unnecessary spinups" - the system is designed to spin-up when data is to be accessed. So from my NAS view, the spinup is necessary. Why? What would you whine if it would not do so in time...?
Moving the thread to the HDD spindown subforum.