It fills the drive with random data so that the previous data can not be
recovered. Now, that is good if you used that hard drive for sensitive stuff
which you don't want anybody to possibly recover. If the hard drive is new
(meaning you never used it or used it briefly or you don't care for the data
on it as it may contain only 56 gb of cats pictures) and I say new for you
referred in your post to a "new computer" then overwriting with random noise
is not something you will want to do for the operation takes a loooong time.
As far as the "Now my PC act as if it is full. What did I do wrong?" I don't
understand and I have no idea.
Welcome to the libre community!
P.S: also notice that hard drive encryption gives you only physical security,
that is security against an attacker that can physically access your PC (as
in the case it gets stolen) and it won't make your PC any more secure on the
net AFAIK..
