Some Samsung Galaxy Nexus owners were surprised when they connected their phones to their computers and couldn’t access the internal storage via the USB mass storage protocol. At first there was confusion about whether this was a problem specific to the or something to do with Ice Cream Sandwich. It turns out this isn’t a problem, but a change in the way Ice Cream Sandwich handles a device’s primary storage.
Dan Morril, an engineer at Google, elaborated on the situation in a Reddit discussion thread. Basically, devices like the Samsung Galaxy Nexus that do not have external storage via an SD slot, will not support the USB mass storage protocol. This is because Ice Cream Sandwich uses primary storage as one big partition instead of separating storage between apps and media. It’s a feature that was introduced with Honeycomb. Since the primary storage of a phone without an SD slot cannot be unmounted (a requirement of USB mass storage protocol), those devices cannot support USB mass storage. Instead, they would have to use Media Transfer Protocol (MTP), or Picture Transfer Protocol (PTP) to access media on internal memory.
For Windows users, this is no problem because both protocols are supported in the OS, making the device look like an external drive. However, Linux and Mac users will have to figure out a different way to work around this since MTP and PTP aren’t natively supported by those OSes.
USB mass storage will still be fully-supported on Ice Cream Sandwich devices with (micro)SD card slots since they aren’t used as primary storage and can be unmounted to be accessed by a computer. Hit up the Reddit thread for more of the nitty-gritty, technical details.