There may be an option within the Sony's menus to switch the mode. There is on my LG M37 player.
Failing that make sure you don't use Windows Media Player (or at least see what options exist within WMP for changing mode).
When I connect this to my computer, it is not listed as a drive. It connects automatically in MTP mode. I cannot drag and drop files to it. And there is no place to change to MSC mode. But it is touted as a drag and drop device. Any help would be appreciated.

Chowhound
Comic Vine
GameFAQs
GameSpot
Giant Bomb
TechRepublic