mount a folder of the usbhd into the default public shared folder
e.g. /home/shared/usbhd
and put something like the following into the smb.conf:
[sharedusbhd]
comment = Public Shared USBHD
path = /home/shared/usbhd
browseable = yes
guest only = yes
writable = yes
public = yes

## in my opinion it should work ...but i do not know whether it's secure or not