file.new text writing securtiy issue.

Ok so I'm working on a project that stores a number of users details and
their associated mailing lists then after all edits writes all the .ali
files to a directory.

I'm using the following code as a test of file.new (Which I haven't used
before).

txtout_file =
File.new("/export/machinename/htdocs/users/public/mailaliases/all.ali","w+")
txtout_file << "Some random text" #write output to the file
txtout_file.close #close the file

This should write the line "Some random text" to the file all.ali, and
it does as long as all.ali already exists. If all.ali is not in the
directory it fails with the following error "Permission denied -
/export/machinename/htdocs/users/public/mailaliases/all.ali"

I was under the impression that the w+ mode would create a file if the
file did not exist. The folder permissions are correct as far as I can
see and yet I cannot get rails to create the all.ali file.

Any ideas?