Can't help on the hardware front unfortunately.
When photos are imported into Lightroom Mobile it will sync the Full Resolution up to the Cloud (you can set 3G or Wifi) then will sync the Full Res again down to Lightroom Desktop when logged in.
Once up in the Cloud, the files on your Mobile Device I believe will be optimised to reduce the space they take up.
On Apple iOS at least the photos are locked in the app. I don't know if on Android you can access the file system but assume it is like iOS. Thus they won't copy to an external disk as it doesn't really work like the Desktop version.
Also it can't sync over a local network.
This has been requested though. For other requests and more info go to the two Lightroom Mobile forums (the user to user one like this and the photoshop family one that is read by Adobe).
Hum That's the thing, The wifi is available once I am back home so when I have access to the desktop.
I am not going to sync Gigs of photos over Data, or over convenience store wifi!
At least JPG would take less space and so I can understand that system, but for RAW no way! And I am lucky I don't live in a country where internet bandwidth is limited and expensive, still upload speed is fairly slow.
So I am better off looking at getting a small notebook and find a way to effectively merge the travel catalog and the main one, that way I don't need to use internet when I can use usb3, and I can do the keywords and metadata.
And I checked, it is possible to access the lightroom version of the photos on Android.
Thanks for your answer