After upgrading my Xperia Ray to IceCream Sandwich (Android 4.0.3), I noticed that I wasn’t allowed an option to connect the phone to the PC in mass transfer mode (USB storage).
The default mode changed to Media Transfer Mode (MTP) which allowed the phone to easily connect with Windows machines, update the firmware, manage files and stuff like that. But what if you wanted to connected the phone to other devices, non-windows based machines?
Well after tinkering around with a lot of options (supposedly deep inside, supposedly restructured after that update), I found it.
Head over to Settings > Xperia > Connectivity > USB CONNECTION MODE > Change the option from MTP to MSC.
Another thing that I noticed from a few forums was that people were actually complaining about this option going reset, especially after you restart your phone, or after ever disconnection from the PC. Well, as far as I have seen, I haven’t noticed anything of that sort.
Hope this helps.