Thanks for the info, I didn't know you could do that with a macbook.@MikhailT
Regardless, it's a far better solution than a low capacity (~128gb) SSD and an Optibay 5400rpm HD.
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I have indeed a 120gb SSD + a 500gb in place of the optical drive in my thinkpad laptop and I really love that setup. It's great for performance and backup.
Might consider this setup for the macbook pro I want to buy next month.
Update: on second thought, I wonder if you could install 2 momentus XT (main + Optibay) and setup your macbook in RAID 0.