In the past I have stored my Lightroom photos on an external FW800 drive. This was necessary due to limited internal drive space on my older iMac. Well, that iMac died and I got a new one with a 2TB Fusion drive (split into 2 1TB partitions). So I now have the space on my internal drive. My Lightroom library and preview files are on the Fusion drive, but my photos are still on the FW800 drive, now connected with a Thunderbolt adapter. I did a little testing, with a small Library on the 1TB internal partition (library file, previews, and RAW files) and did not see any difference in performance between that library and my main library that is pulling the RAW files off the external. This was a quick test and I was mainly just watching how long it took to switch to the Develop module and cycle through images with various amounts of edits applied. It would basically take anywhere between 1/4 and 3/4 spin on the spinning progress icon regardless of which Library I used. So, my anecdotal evidence is that there is no real advantage to storing the RAW photos on the internal drive. Is this consistent with other users experiences? I prefer to keep the files on the external because they really are the most important files on my computer and, after suffering from a recent hdd crash on the old iMac, I like just being able to plug the drive back in and not have to worry about a TM restore. Of course, there is nothing to say that my external hdd couldn't crash, but it gets a lot less use than the internal since it doesn't also host the OS and all my other files, so I feel the odds are better. Either way, I do have the TM backup in case.