Macs have the concept of “flags” which live alongside filesystem permissions for a file. In particular they have a “read only” flag that trumps whatever the filesystem says a user’s or group’s access is supposed to be. If this flag is set on a file, you cannot rename, delete, or edit that file even though you would be able to on a Windows or Linux machine. Other BSD based systems also respect this flag including the Isilon itself which is why it matters for DartFS.
The reason this should be a FAQ (or at least an OAQ) is that there are
Mac tools that set this flag. RCS (version control) is one example and
there may be others.
On the Mac, you can see if this flag is set by using the -O option to
‘ls’. It will show up as ‘uchg’. To manually remove it, use
‘chflags nouchg file’