I have the same problem. Here are the details:
External Hard Drive with LAN (Buffalo Drivestation Flexnet:
http://tinyurl.com/kk5mmu)
Mac OS X 10.6, on 3 different Mac Mini's
This error occurs on all computers when trying to copy a file to the 'SHARE' sharepoint on the SMB Drive:
"The Finder can't complete the operation because some data in "filename" can't be read or written. (Error Code -36)"
I CAN
Access / Read / Write this Drive in Windows XP
Access / Read / Write this Drive in OS X 10.5.7
Make new folder on this Drive in OS X 10.6
See a .DS_Store file which 10.6 has written to the Drive
Save a document to the drive from the Text Edit save dialog in 10.6
Save a document to the drive from the Pages save dialog in 10.6
Duplicate a document on the drive using Terminal (cp Untitled.pages Untitled2.pages) in 10.6
Copy anything to any other computer on the Network using 10.6
I CAN NOT
Copy a document to the Drive in 10.6
Duplicate a document on the Drive in 10.6
Save a PDF or JPG to the drive from the Preview save dialog in 10.6
This error happens every time. It is easily reproducible.
I hope that this information helps Apple get on to the problem soon.