Vista Networking Corrupting my Data!

I almost don't know where to start, since most of my other posts with
previous problems have gone unanswered. Let me just add the latest.

I have a vista laptop connected to a XP desktop with a crossover cable. No
wireless. XP connected to broadband modem. I have an external HD connected
to the desktop that is having data on it corrupted when I try to transfer
from the laptop to the LACIE external drive on the desktop via the network.

Prior to this I have been able to download data to this HD, but the last two
times I have downloaded photos from my Olympus camera to the external HD via
the laptop and network, it has copied some of the files, then given me a
message saying there was an unexpected error and the process cannot continue
due to "insufficient quota". Then the files it has copied onto the HD become
corrupted. It has somehow changed the Jpeg markers in the files so that
nothing will read them anymore. This is incredibly annoying and frustrating.
WHY DOESN'T WINDOWS NETWORKING EVER WORK PROPERLY????

The less annoying but recurring problem that I never received any answer to
I will post again here:

Vista laptop connected to XP desktop by crossover cable. XP machine
connected to internet and to external HD. I can access the XP's printers,
share files, etc (most of the time) but there are problems with the external
HD. It keeps disappearing from the network (from the laptop's perspective).
Mind you, the network itself only ever works if I turn the XP desktop on
first. Sometimes when I turn the laptop on the rst of the network is
connected fine but the LACIE external HD has disappeared and I have to go
through the process on the XP machine of sharing it again. At other times I
turn the laptop on and it is there just fine. I don't change anything. Why
does Windows do these mysterious annoying things? Is it that there has been
some new update downloaded that has reset something? How do I stop it doing
this? Why should I have to reshare the external HD whenever I boot up again?
(or more precisely randomly sometimes when I boot up) Isn't there some way I
can say to Windows, "hey listen, I really really mean it - I want this thing
to be permanently shared, don't ever unshare it again until I tell you!!"?