I had the same problem setting up my husband's new Dell laptop. Windows 7, 64 bit. I personally use a Mac. Couldn't understand why I could see and access files perfectly with my Mac and all of our old PCs running Windows XP. In fact I am running a virtual machine in my Mac with Windows 7 (32 bit) and it works fine.
I got the run around from Dell, told me to call Microsoft. Microsoft told me to call Dell, after trying to blame the router. Long story went nowhere.
Finally with the help of a quick assistance code (eternally grateful to my son-in-law who works at Microsoft) I got some assistance.
First as I kept reviewing all my networking settings in Windows 7 machine I mapped the drive (Z
changed the encryption from 128 bit to 40//50 bit. I think that is what allowed me to access the WD MyBook World without having to login every time. I could then see and open files. However, I still was not able to save files (access denied) or even create a new one and save it to the drive.
What Microsoft tech did for me was to go to a Dos prompt and grant rights to essentially everything on my Z drive. Didn't get whole command, but it was something like this Z: icacls *.*/grant everyone: f/t/c It successfully processed 23, 163 files and failed to process 4,733. Whole process took probably an hour and a half.
Hope this is helpful to others struggling out there with similar issue.