How to save a file quickly and no need to wait for the external hard drives to "wake up"? Recently I have decided that since Mac does such a nice job for indexing the files, I will store many files on the Mac, so I bought a 6TB Thunderbolt external drive which is in RAID 0 Strip mode, and another 2 external drives that are 4TB each, and use them as the RAID 1 mirror mode.

The iMac is 1 year old, so it is still using USB 2.0, but the drive using Thunderbolt is fast.

However, if I just right click an image on a website and choose "Save Image As", now I have to wait 30 seconds for the drives to "wake up", before the "Save As" will pop out the window to let me click the "Save" button at all to save the file.  Since some Mac can now boot up in 25 seconds, I need to wait a longer time than the boot up time just to save an image, which 99.99% of the time is by default just on the Mac's internal hard drive, under ~/Downloads.

Is there a way to improve this?  I also turned off the "Put the hard drive to sleep when possible" in the System Preferences, but it is still doing the same thing.  For example, if the "Save Image As" can let me choose the "Downloads" folder, and just to wake the other drives up when I do select to look into them.

One method I can think of is, write a shell script that will ls /Volumes/WD6TB and ls /Volumes/WD4TB (the drives' names), just to wake the drives up whenever I start using this iMac.  Or else if there is a way to cause the screensaver to wake up the drives whenever there is a mouse move or keyboard action, wake the display (which it can do), and wake the hard drives, that will be a good solution too.
The save button is checking all mounted drives, and will spin them up.  

Your only way I can think of to do this is to prepare a couple of scripts.

See the man page for diskutil.  If the mount point (diskname) is BigFatDisk

then to unmount the disk it would be something like

diskutil unmount BigFatDisk

Now to keep everything sync'd to BFD you would run a script now and then

diskutil mount BigFatDisk
rsync flags local disk BFD
diskutil unmount BigFatDisk

A second option is to replace the disks in the BFD enclosure with hybrid disks.  These have an SSD chunk and a spinning rust chunk, and are ideally suited for this.

A third option is to let your disks spin whenever the machine is awake?  The amount of power they use is small, and increasingly MTBF times are being given as X spinups, not X hours.  A hard drive 2 years ago used 5-7 watts.  I'm sure it's better now.  So let's use 5.  That works out to 40 kWhr per year, roughly.  At 15 c per kWh that's 6 bucks/year.  Seems to me that this is pretty cheap compared to the hassles you have now.

I have set my computer to turn the screens off after 20 minutes idle time, but never to turn off the CPU or spin the drives down.