It has been discovered that although Samsung’s Galaxy Nexus smartphone does not support USB mass storage for mounting the phone as a drive on a computer, the OS behind the phone, known as Android 4.0 Ice Cream Sandwich, does support the feature. According to Reddit commenter morrildl, the reason that the Galaxy Nexus does not support this feature is because the phone launch without removable storage, and the feature would only be enabled for devices with separate partitions for apps or with removable memory card slots:
It isn’t physically possible to support UMS on devices that don’t have a dedicated partition for storage (like a removable SD card, or a separate partition like Nexus S.) This is because UMS is a block-level protocol that gives the host PC direct access to the physical blocks on the storage, so that Android cannot have it mounted at the same time.
As the Galaxy Nexus gives users and apps the entire amount of storage on the device, rather than partitioning a separate area for storage of apps, the unified storage model means that you can’t just plug and play your smartphone into a PC and mount it as a drive.
However the cost is that Android can no longer ever yield up the storage for the host PC to molest directly over USB. Instead we use MTP. On Windows (which the majority of users use), it has built-in MTP support in Explorer that makes it look exactly like a disk. On Linux and Mac it’s sadly not as easy, but I have confidence that we’ll see some work to make this better.