A few things you need to look at as long as the file runs fine when you run it.

1. What user is the SQL server agent running as? If the tape drive is not local you will need a user that can access the tape drive.

2. If it is running as a user log into the machine as that user and see if the script works.

The command shell will run with the authority of calling user so you probably need to look there. Use the configuration manager to change it and this should clear it up.

I have also found that making sure SP2 is installed as this has solved a lot of problems for me.