What could be causing my hard drive to spin down?

Question

Recently I noticed that I could hear my hard drive spinning down very frequently, like once every 10-20 seconds. At that time spin down was set to 30 minutes using the following udev rule:

hdparm -B 127 -S 241 /dev/$kernel

This was confirmed by hdparm:

$ sudo hdparm -I /dev/sda | grep level
        Advanced power management level: 127 

So I disabled power management with

$ sudo hdparm -B 255 /dev/sda
$ sudo hdparm -I /dev/sda | grep level
        Advanced power management level: disabled

But the drive is still spinning down frequently:

$ sudo smartctl -a /dev/sda | grep Load_Cycle_Count
225 Load_Cycle_Count        0x0032   100   100   000    Old_age   Always       -       8263
$ date
Mon  7 Dec 16:08:06 GMT 2015
$ sudo smartctl -a /dev/sda | grep Load_Cycle_Count
225 Load_Cycle_Count        0x0032   100   100   000    Old_age   Always       -       8265
$ date
Mon  7 Dec 16:08:16 GMT 2015

(I know this spin down count is low, I've just moved the installation to a new drive. I assume that the issue was the same with the old drive, but I just didn't notice. The cont on the older drive is 284,572 which I installed six months ago.)

What could be causing my hard drive to spin down and what can I do next to investigate?

Answer 1

R00KIE on the Arch Linux Forums correctly suggested that I try hdparm -B 254 /dev/sda. This has stopped drive spinning down so often, and the drive now obeys my hdparm -S 241 /dev/sda setting.