You are using the wrong option, -y forces the drive into a low power state, try using -S which tells the drive to spin down after a specified amount of time of inactivity. If you were to use -Y option your drive would work as your expecting it to act, but the result is an immediate spindown. I use this for my mirror drive on my home NAS and it works like a charm.
The command you want would look like this:
sudo hdparm -S 240 /dev/disk/by-uuid/f702eda2-2ebe-4aa6-bad8-da80e5f2b355
Here is the description of the option:
-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.
and the page where i found it is here:
Two heads are better than one, unless one's a goat head.