The tape has a physical write protect mechanism. On 3490s there's a thumbwheel 
on the tape and if you turn it until the flat surface shows, then it's write 
protected. Other types of tapes such as 8mm Exabytes may have break-off tabs 
like those on music cassettes.

Also check the access of the tape - e.g. q vol tape01 f=d.

This will show if the access is readonly.

Regards

Alex. Todd