This week, Other World Computing published a series of guidelines for the owners of the latest MacBook Pro models, making it clear what are the options for expanding storage over SATA connections that Apple decided to implement in the first batch of professional laptop units, updated on this year.
The models manufactured until a few weeks ago had version 3.0 of the interface in the HDD bay (although firmware problems prevented it from working correctly in some cases) and version 2.0 on the optical drive, but now the Mac Performance Guide suggests that Apple has changed its production scheme to implement the latest SATA version on all models, in both bays.
The change was seen by the 9 to 5 Mac and their sources, but that still doesn't guarantee that high-speed SSDs will work, due to compatibility issues. Such problems are yet to affect users who purchase 17-inch MacBooks Pro, but they can be resolved as a firmware update that Apple could release in the future (again: it doesn't mean to go).
To check the speed of your laptop's SATA connections and see if they are 3.0 or 2.0, just open the System View (System Profiler) through Spotlight. The image above (click on it to enlarge it) shows what should appear when clicking on the Serial-ATA option (located in the sidebar of the utility window) on a newly manufactured MacBook Pro.