These two options seem confusing. For example: according to the man page -B 254 "does not permit spin-down". However, testing with -B 254 -S 1 the drive does spin down after 5 seconds.
-B Query/set Advanced Power Management feature, if the drive supports it. A low value means aggressive power management and a high value
means better performance. Possible settings range from values 1
through 127 (which permit spin-down), and values 128 through 254
(which do not permit spin-down). The highest degree of power
management is attained with a setting of 1, and the highest I/O
performance with a setting of 254. A value of 255 tells hdparm to
disable Advanced Power Management altogether on the drive (not all
drives support disabling it, but most do).
-S Put the drive into idle (low-power) mode, and also set the standby (spindown) timeout 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.
