If I understand you correctly the USB disk drive is left mounted on
your Lucid Ubuntu machine and you are trying to access it over the
network from your Windows 7 machines. If you have Samba installed
correctly it should work but you will have to add the USB drive
manually as a shared drive to the configuration tab of gsamba-admin.

My smb.conf includes:

[Disk-1]
comment = disk-1
path = /media/4A42-57A5
; ^== name of drive mounted as a folder in
the Media folder.
read only = no
available = yes
browseable = yes
writable = yes
guest ok = yes
public = yes
printable = no
locking = no
strict locking = no

This is not necessarily 100% correct, but it usually works. Yet
sometimes I find that I have to go into gsamba-admin and deactivate
then restart the network before it works properly. That then requires
manually running winbindd as root. I also find I have to reset the
password for my account with smbpasswd -a [username] to get full
visibility and sharing going.

I've never found out why this is necessary, but I'd certainly like
some advice on this! I've asked about this anomaly several times and
nobody has responded with anything helpful. I am running Kubuntu 10.04
and 10.10, with one Win XP machine. On a good day everything is
shared, visible and writable everywhere - but there are bad days which
is frustrating because I've never been able to find out exactly why.