MySQL "SELECT INTO OUTFILE" fails with CIFS-mounted volume

SELECT INTO OUTFILE fails for a CIFS-mounted directory (/mnt/backup/test) but works for a normal directory (/tmp/tests).

mysql> select * from mytable into outfile '/tmp/tests/mytable.txt';
Query OK, 3723 rows affected (0.05 sec)

mysql> select * from mytable into outfile '/mnt/backup/test/mytable.txt';
ERROR 1 (HY000): Can't create/write to file '/mnt/backup/test/mytable.txt' (Errcode: 2)

The mysql user has no problem writing to /mnt/backup/test/mytable.txt :

root:~# su - -s /bin/bash mysql
mysql:~$ touch /mnt/backup/test/mytable.txt
mysql:~$ ls -l /mnt/backup/test/mytable.txt
-rw-r--r-- 1 root root    0 2009-11-16 10:48 /mnt/backup/test/mytable.txt

The CIFS directory /mnt/backup is mounted with : rw,noperm,user=****,password=****

Since the -T option of mysqldump makes use of it, what options (Mysql/CIFS) are needed for SELECT INTO OUTFILE to work with my CIFS-mounted directory ?

Update:

Interestingly, an strace of the mysqld process did indeed reveal a access control failure as Michael suggested. But I still don't know why. A simple Perl script launched as mysql calls the same syscall with no problem.

Extract of mysqld strace

open("/mnt/backup/tests/bugs.1.txt", O_WRONLY|O_CREAT|O_EXCL|O_LARGEFILE, 0666) 
        = -1 EACCES (Permission denied)

Extract of perl strace

open("/mnt/backup/tests/bugs.32064.txt", O_WRONLY|O_CREAT|O_EXCL|O_LARGEFILE, 0666) = 3

Note that the directory /mnt/backup/tests was emptied before each run.