Spare disk periodically woken up

Hi there.

I'm very sorry if this is an FAQ; I've searched the wiki but have not been able to find the answer.

I have five disks, four of which form a RAID5 set, with the fifth being a spare. This fifth disk is normally spun down, or at least it should be. (It's a bit more complicated than that, as is evident from the info included below, but this is essentially it).

I recently noticed that this fifth disk is periodically spun up for reasons unknown. It's not due to SMART monitoring because I've disabled the SMART daemon altogether.

My /proc/mdstat is shown below; the disk that is periodically spun up is /dev/sdf.

How can I find out what is causing my standby disk to spin up?

Many thanks.

Cheers, Jan

========= /proc/mdstat
Personalities : [linear] [multipath] [raid0] [raid1] [raid6] [raid5] [raid4] [raid10] md2 : active raid5 sdf2[4](S) sda2[0] sdd2[3] sdc2[2] sdb2[1]

      934765824 blocks level 5, 64k chunk, algorithm 2 [4/4] [UUUU]

md1 : active raid1 sdb1[0] sdd1[1]

      979840 blocks [2/2] [UU]

md0 : active raid1 sdf1[2](S) sda1[0] sdc1[1]

      979840 blocks [2/2] [UU]

unused devices: <none>