I have recently started preparing my older Mac Pro 4,1 (Early 2009) 8-Core Dual 2.26MHz for switching OS X from Snow Leopard 10.6.8 to El Capitan 10.11.6. I have a new Samsung EVO 1TB SSD drive on the way for the install of the new OS. I also downloaded the El Capitan Installer and saved on my current OS drive. Right now my current boot drive (Bay-1) is running Snow Leopard (and mind you beautifully!). I was wondering if I could just pull out the existing 1TB drive from Bay-1 and slide it into Bay-2 - Install the new bare SSD into Bay-1 - Restart the machine and then install 10.11.6 from the install file on SL by selecting the new SSD when prompted for a destination drive. My MAC Pro will automatically find the original boot drive now in Bay-2 and just boot to it right? If this works my plan would be to "option" boot into each OS at will while I get acclimated to El Capitan. Later I would probably wipe out SL in Bay-2 and use as a file storage drive once I know I am 100% done with SL…although sadly..it will surely be an OS I will forever miss I must comment here that it has worked almost 99.2% flawlessly for me over the past 7 years up to present and will forever be an OS I will remember as one of the most rock solid and best performing I have ever had the pleasure of living in...I have literally no major complaints about it and have loved it all this time.