Will internal HDD spin down when a Mac is booted from another internal SSD?
I had this setup for a while and yes, the hard drive in the primary disk bay would spin down when booted from the SSD. It was fairly easy to tell, because when accessing the drive there was a noticeable delay when first accessing the drive after leaving it idle for about 10 minutes. This was on an older machine (2009 MacBook Pro 15", running Mac OS X 10.7).