This is not a driver issues, more likely what you are seeing, is a SMB (Samba) issue.
(FYI, the DNS-321 uses Samba to allow Microsoft OSs to access the DNS-321's drives)
In Windows 7 (beta) Microsoft has "re-worked" their networking component to make it easier to
share you files with other Windows 7 computers. Part of Microsoft's new networking code is a new
concept called "HomeGroup". from http://www.neowin.net/news/main/09/01/13/windows-7-homegroup-overview
"HomeGroup in Windows 7 will simplify the tasks associated with sharing music, pictures and documents
within your home network and Windows 7 PCs."
Problem is, this new networking concept doesn't seem to work well with non-Windows 7 (beta) PCs.
Perhaps, there will be new code introduced in a future version of Samba (www.samba.org) to account
for this new networking concept, if Microsoft is forthcoming with the detail of their new networking strategy.
See http://www.tomshardware.com/forum/75-63-windows-samba-issue for a discussion of Win7 Samba issues.
Until, Samba is updated, and D-Link uses the newer Samba in the DNS-321 firmware, you should not
expect Windows 7 (beta) PCs to work well with the DNS-321.