man hdparm
-S Put the drive into idle (low-power) mode, and also set the standby (spindown) time\u2010
out for the drive. This timeout value is used by the drive to determine how long
to wait (with no disk activity) before turning off the spindle motor to save power.
Under such circumstances, the drive may take as long as 30 seconds to respond to a
subsequent disk access, though most drives are much quicker. The encoding of the
timeout value is somewhat peculiar. A value of zero means "timeouts are disabled":
the device will not automatically enter standby mode. Values from 1 to 240 specify
multiples of 5 seconds, yielding timeouts from 5 seconds to 20 minutes. Values
from 241 to 251 specify from 1 to 11 units of 30 minutes, yielding timeouts from 30
minutes to 5.5 hours. A value of 252 signifies a timeout of 21 minutes. A value of
253 sets a vendor-defined timeout period between 8 and 12 hours, and the value 254
is reserved. 255 is interpreted as 21 minutes plus 15 seconds. Note that some
older drives may have very different interpretations of these values.
-y Force an IDE drive to immediately enter the low power consumption standby mode,
usually causing it to spin down. The current power mode status can be checked
using the -C flag.