I'm going to add a SSD drive to my macbook pro. I'm wondering if the optical drive adapter can have any influence on the disk speed, taking it down from a SATA III to a SATA II or I.
Regarding SATA I, II and III and getting the best speeds speeds — it really depends on what specification your machine is, and what adapter you get.
I would suggest you install the SSD in place of your original HDD, that way you don't lose out on the SSD's potential because of an adapter:
In either case, I would install the SSD there, and then install the HDD in the optical bay, because:
To find out what you have, go to Spotlight (the magnifying glass in the top-right of your screen), and type "System Information", open that app, go to