Even before a possible shortage of storage I understand the problems to be ...
a. don't know what Linux distribution;
b. no optical drive;
c. can't get an external optical drive to work.
For Asus Eee pc 900 with only Linux installed I found
this. If that is what you have then I'm pretty sure you have
Linpus Lite.
I took a quick look at Linpus Lite when I read that Asus was using it on their netbooks. I don't care for it, but it seems fairly simple to use for "standard" things like web browsing and word processing.
If I recall correctly I had difficulty finding a Terminal ("Command Prompt") window. Maybe in the Administration module--at least there is supposed to be a manual in that section.
Does the external drive have drivers or software for Linux? If so, for all distributions or just some?