A feature that the Sony design team chose not to include in their early 2012 flagships was mass storage mode. While MTP has its advantages over USB mass storage, this exclusion left many users scratching their heads. Not only is USB mass storage faster, it also wouldn’t really hurt to have the choice. But all things aside, there’s now an app called SD Mounter by XDA Senior Member Soheil_rf that enables mass storage developed on the Xperia S, P and U running Ice Cream Sandwich and Jelly Bean.
Based on XDA Recognized Developer DooMLoRD‘s SD Card Mounter for the same devices with Gingerbread and XDA Senior Member Arakmar‘s scripts, SD Mounter allows users to easily activate either mass storage mode or media transfer mode. A rooted device with busybox is required for it to operate properly, so don’t be surprised when SuperUser prompts for root permission. As with any mass storage device, it is also recommended to eject your device from your PC once you’re done your business before you switch back to media transfer mode to prevent any unwanted storage errors.
So if this interests you, be sure to check out the original thread for more information.