USB Mass Storage is the old method and it shows up as a drive. The Droid X is older than the Xoom therefore it is using USB Mass Storage.
MTP is Media Transfer Protocol and is the method used by 3.0 and higher.
How is it that you're using Linux and you don't understand this stuff?
...Android 3.1 still isn’t intended, at the operating system level, for managing external storage devices. This detail, which came out in conversations I had with Google engineers today, explains why the Android file system organization is, well, a mess...
“We don’t want to expose the user to file locations,” explained Hiroshi Lockheimer, director of engineering at Google. “How do you manage that as a user? The not-good answer is with the file browser.” Lockheimer says that many of the experiences I’ve seen so far are what individual hardware manufacturers have enabled, as opposed to what Google has provided natively. The good thing is that Lockheimer says Google is looking at ways to do this; but the problem hasn’t been solved yet...
He did answer you. The xoom uses mtp which will not show up as a drive in Linux unlike older android which used mass storage. Mtp allows you to add files with out having to unmount the file system on the xoom. There are many threads on how to set mtp up. If you're new to Linux it may seem hard to do but its simple. Do a Google search for mounting xoom Ubuntu and you should find them easy.
I can mount my Xoom in linux using libmtp and mtpfs, however, it's slow, kludgy, and generally feels like it will flake out at any moment. Additionally, even when it's mounted, I can't seem to copy music across properly; it doesn't keep the folder structure. In fact, the directories are created and any non audio files are copied across fine, but every mp3/audio file is flatly dropped into the Music directory.
I tried to copy some music onto it from my fiancee's windows xp laptop and it worked ok (though the album art didn't appear).
The move away from mass storage to MTP is a massive disappointment for me. Mass storage has always been a big selling point for me when purchasing a device and for Google to switch to using a Windows specific protocol and consider it cross platform is a big letdown. MTP on mac requires a special app and on linux, support is dire, certainly for anything more complex than simply pulling images from a camera.
I just want to clear up a misconception about MTP. It is not a cross platform solution for transferring files.
My post from another thread on the subject:
I can just about get files onto the xoom, but I can't load music onto it; all of my mp3s just land up in the top level of the Music directory, regardless of how I attempt to transfer the files. I think transferring a zip file of the music and unzipping from within a file manager on the xoom might do the trick, but this isn't a good, workable solution. I understand the reasoning behind MTP, but it most certainly isn't cross platform and shouldn't be mentioned as a working solution by Google.
Hopefully people will soon realise not to keep insisting that the xoom has linux support when using MTP.