One major change introduced by Google in Ice Cream Sandwich was the use of MTP to transfer files and a unified memory storage structure In the pre-ICS era, Android phones had their internal memory divided into two partitions. One for installing apps, and the other one for all your storage needs.
With ICS and Galaxy Nexus, the whole internal memory was unified. There were no more partitions, which meant that the whole 16GB (13.3GB actually) could be used for installing apps or for storage purposes. While all this sounds good on paper, the use of MTP to transfer files has created a big nuisance for all Android users. Not only does MTP takes ages to transfer files, it does not work natively on Mac and Linux.
While HTC went back to the USB Mass Storage mode with the One X, Samsung decided to stick with MTP. While it was not possible to enable USB Mass Storage mode on the Galaxy Nexus, the Android community has managed to enable USB Mass storage mode on the Galaxy S3.
The USB Mass Store hack will only work for the external SD card of the handset, and not the internal memory of the handset. While the hack has been available for quite sometime, it required users to enter some commands using Terminal emulator on their handset. An XDA member, Kopfgeldjaeger, has written an app that allows users to easily enable USB Mass Storage mode on their Galaxy S3. The app works with the International as well as the U.S variants of the S3, but does require root access.