Just wanted to follow up on this.
The problem was that I was attempting to use a soft link to redirect the default zimbra backup folder to a folder under a mounted fs:
/opt/zimbra/backup -> /mnt/nas5000/zimbra_backup_folder
The correct way to accomplish this is to specify "/mnt/nas5000/zimbra_backup_folder" as a target using the -t option for zmbackup. I removed the link and configured the backup target in the Zimbra crontab:
0 1 * * 6 /opt/zimbra/bin/zmbackup -f -a all -t /mnt/nas5000/zimbra_backup_folder
0 1 * * 0-5 /opt/zimbra/bin/zmbackup -i -a all -t /mnt/nas5000/zimbra_backup_folder