Set the standby (spindown) timeout for the drive.

This 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 "off". Values from 1 to 240 specify multiples of 5 seconds, for timeouts from 5 seconds to 20 minutes.
Values from 241 to 251 specify from 1 to 11 units of 30 minutes, for timeouts from 30 minutes to 5.5 hours. A value of 252 signifies a timeout of 21 minutes, 253 sets a vendor-defined timeout, and 255 is interpreted as 21 minutes plus 15 seconds.