When Apple rolled out the new MacBook Pros with Thunderbolt I/O, the company revealed a SATA3 interference for the hard drive and left the Superdrive in SATA2. SATA3 allows for theoretical data transfer speeds of 600 megabytes per second versus 300 megabytes per second for SATA2.
However, multiple sources are now claiming that Apple has upgraded the recently delivered units to SATA3 connectivity in the optical bay. This does not mean that the latest SATA3 SSDs will work in the optical bay due to compatibility problems with the SATA interface. Most likely it will require an EFI update, or another small hardware adjustment.
Sites such as Other World Computing and Mac Performance Guide have both reported on having 6Gb/s in both the main drive and the optical bay.
To see whether your MacBook Pro supports SATA3 in the optical bay, load up Apple System Profiler app and examine the Serial-ATA entry under the Hardware tab. If the link speeds is 6 Gigabit, you're in luck.
via 9to5mac