Hello Freeuniverse,welcome to trisquel and gnu/linux. Sorry this will not be an answer as I do not know how to do it. But I have two questions:
when creating the freeusb did you leave some free space? if you did not then when you do find out how to do it it will not be saved.
why do you want to hide it? the drive does not mount unless you tap on it. it just indicates it is there.
