Backup fail with errno9 Bad File Number

One of my instances is intermittently failing to backup (using ontape, level 0) and giving me 9 EBADF Bad File Number as the error. It always fails after 20 percent done. I have switched tape devices and it still fails (intermittently). Informix asked me to bump up NFILES. I used the formula in the book assuming it is PER INSTANCE and bumped it up to 512. It didn't fix the problem. Has anyone had this error before using ontape? Does anyone know if NFILES is per instance or is an accumulation of all instances? (I start all instances with one /etc/init.d script.) The log from backup script is as follows: ######################################### Starting backup of Informix instance ncciboca at Sun Sep 8 22:10:55 EDT 1996 Using device: /dev/rmt/2 Rewinding tape... Backing up Informix instance ncciboca Please enter the level of archive to be performed (0, 1, or 2) Please mount tape 1 on /dev/rmt/2 and press Return to continue ... 10 percent do ne. 20 percent done. Archive failed - function write to tape failed code -1 errno 9 Program over. Informix Backup of ncciboca finished at Sun Sep 8 22:32:28 EDT 1996 ######################################### TIA, Kathy