I have "Disk spindown idle time" set to 300 in version 378.55 of the Merlin FW on an RT-AC68U. In the system log I see "sd-idle-2.6[4469]: spinning down /dev/sda after 5 mins 30 secs" entries, but after each one there is a "sd-idle-2.6[4469]: spinning up /dev/sda after 4 mins 31 secs" entry. The disk behaves as you would expect with these log entries, spinning down after about five minutes and then spinning up again about five minutes later. It seems that something might be accessing the...
Spinning down .... Spinning up
Spinning down .... Spinning up