Problem with the new Storage module

It looks like your issue is f.write("a","Hello",0); -Storage assumes that if the offset is 0 you intend to write a new file, so it just creates a new file with 5 characters.

Instead, just combine the first write and you're sorted:

f.write("a","Hello",0,20);

How to determine the size of a file without reading it?

http://www.espruino.com/Reference#l_Storage_read says:

This function returns a String that points to the actual memory area in read-only memory, so it won't use up RAM.

Basically running f.read("a").length is just as efficient as if there were a special-purpose length function, so we just saved some space and left it out.