Re: Should you keep HDD running
If you have no cost constraints, keep'em spinning :P You can save energy by letting the disks spin down. If they are 7200rpm disks they will use 6-9W when idling, or 0.5W when spinned down. Green drives running at ~5400rpm do not need to be spinned down; they will idle at 3.5W which is quite low already.
It also depends on other factors, like how often will those HDDs be accessed. The sequential speeds are more dependent on data density than on spindle speed, and random IOps. I'm guessing you are using the SSD for system and HDDs for lage files thus sequential performance is the only relevant stat here.
You might want to think about spinning down after a few hours of inactivity.