Can't see external hard drive on network

I have a desktop running Vista and a laptop running XP networked to each  other.  I can see all devices on the network except the external hard drive, which I formatted last night.  I have shared the HD and given permission to everyone, but still cannot see it.  How do I add it to the network?

 

I used to be able to see the external hard drive, but could never back up to it from the laptop.