On an Asus RT-N16 I have a Seagate ST9808211A drive connected via USB 2.0. The log shows that it detected an extended idle condition and issues the spindown command but the drive doesn't spin down. No errors are seen on the log file:
user.info sd-idle-2.6: spinning down /dev/sda
According to the Seagate site this drive support sleep mode. This unit a PATA drive and not SATA.