Your NAS is just fine, smbfs just was buggy. I've located the bug and fixed it. Expect smbfs 1.74 shortly.
Here's a diff for the impatient (anyone building smbfs by themselves): http://sintonen.fi/temp/smbfs-writebraw-fix.diff (the patch applies against smbfs 1.73 available from http://amiga-smbfs.sourceforge.net/ )
Anyone blaming QNAP for these "problems": This was NOT their fault. Any system running fresh samba had the same problem.
