TeX Live (which I'm guessing you're using) normally doesn't allow file write operations outside of the current directory path because it's a security risk. This is governed by the
openout_any setting, which may take one of three values (information copied from
a (any) : any file can be opened.
r (restricted) : disallow opening dot files
p (paranoid) : as
r and disallow going to parent directories, and restrict absolute paths to be under
Normally the setting is
p, which doesn't allow going to parent directories. I really don't recommend changing this setting.
You might want to consider changing
$TEXMFOUTPUT to the target directory (since you're using an absolute path), which should then be allowed write access under TeX (if the filing system permits it), but I think David's comment is better:
it is far safer to keep the security setting but make a symbolic link
in the current directory to your global file so you can write that
file with a local path
If you are absolutely sure that you want to lower the security settings, you can change your preferred setting in your
texmf.cnf file. But I really don't recommend it.