This is an explanation from Android Engineer, Dan Morrill. This is from DroidLife:
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And the last part of this mega-G-Nex post has to do with the lack of USB mass storage on the device. Shocked to hear that? You aren’t the only one. To explain his team’s move, Android engineer Dan Morrill took to
reddit to discuss:
"ICS supports USB Mass Storage (UMS). The Galaxy Nexus does not. This is the same scenario as Honeycomb, as for instance HC supports USB Mass Storage while Xoom does not.
If a given device has a removable SD card it will support USB Mass Storage. If it has only built-in storage (like Xoom and Galaxy Nexus) it will (usually) support only MTP and PTP.
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.
With the unified storage model we introduced in Honeycomb, we share your full 32GB (or 16GB or whatever) between app data and media data. That is, no more staring sadly at your 5GB free on Nexus S when your internal app data partition has filled up — it’s all one big happy volume.
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.
On the whole it’s a much better experience on the phone."
And he didn’t stop there. If you cruise over to
this reddit thread and search for user
morrildl, you can find a variety of responses to questions from other reddit users. He talks file systems, SD cards, why some phones leave out external storage slots altogether, and more. Great reads.