Mid 2012 MacBook Pro´s Optical Bays sata port can´t handle SATA III (6Gb/s)!!! (I have MacBookPro9.1 mid 2012 which is June 11th release)
I´ve tried everything and ended up buying WD Scorpio Black SATA II 750GB 7200RPM-drive. Now my new Samsung 840 PRO 256GB SSD-drive is in the main HDD-bay and Sorpio Black in the optical bay. Both are bootable as i installed both with Mountain lion. Everything works finally!
If you wan´t to use your old Itunes folder, import your old system with Migration Assistant or restore when installing Mountain lion to the WD Scorpio Black-drive.
Then after your SSD has been installed and booted up press option key and click Itunes and the you can choose the library. Go to your other drive and navigate to the Itunes-folder.
go to system preferences -> Power management and then check the put harddisks to sleep when possible.
Then go to terminal (applications/utilities)and change the idle drive sleeptime:
sudo pmset -a disksleep VALUE
I have set VALUE to 1 and the Optical bay hdd spinsdown after a minute its been idle and ofcourse spins up when its needed.
CHECK THIS THREADS 1ST POST for more good tweaks:
http://forums.macrumors.com/showthread.php?t=1278494