I was having the same issue. Fifteen disks all spinning up immediately after spinning them down. I would see the following in the logs:
Jul 1 08:09:27 Tower emhttpd: spinning down /dev/sda
Jul 1 08:09:49 Tower emhttpd: spinning down /dev/sdd
Jul 1 08:09:51 Tower emhttpd: read SMART /dev/sdd
Jul 1 09:09:28 Tower emhttpd: spinning down /dev/sda
Jul 1 09:09:52 Tower emhttpd: spinning down /dev/sdd
Jul 1 09:09:54 Tower emhttpd: read SMART /dev/sdd
Jul 1 10:09:29 Tower emhttpd: spinning down /dev/sda
Jul 1 10:09:55 Tower emhttpd: spinning down /dev/sdd
Jul 1 10:09:57 Tower emhttpd: read SMART /dev/sdd
I would also notice about 20-40Kb/s read rate on each drive.
As soon as I disabled Turbo write, the issue stopped. Read rate is now 0B/s, and drives stay spun down.