Networked computer will not access usb hdd

Hello,






Main computer running win7 and has usb hdd attached. 2nd computer running xp and networks well with homegroup networking. However I have installed a samsung 1tb usb hdd on the w7 pc which is recognised by the 2nd xp pc but cannot access it. An error message appears asking for administrator. I am the administrator and have ensured that sharing access is granted but it still will not allow me to access. Any suggestions will be appreciated