> Once I considered this approach, but problems was that
> nFileIndexLow/High can change every time file handle is opened, so it's
> not unique. See remarks in following page.
> http://msdn.microsoft.com/en-us/library/aa363788%28VS.85%29.aspx

Actually, that page only says that file identities may vary over time,
and that it may happen during operations such as defragmentation.
However, I have been in contact with a customer who observed a file
system where the file index actually changed every time a file was
closed and re-opened, given that nobody else kept the file open
inbetween. This was on some kind of network mount, I wasn't told which
kind (and I couldn't reproduce it with samba).

This means that it's actually essential that you don't close the file
between the CreateFile calls (and yes, your patch does this correctly).