Hi Simon,
This is a process of determining where the root of the problem lies. If you plug the external drive into a PC or Mac, is it seen by the PC/ Mac? Can you read a CD or DVD on the PC/ Mac? If that is OK then your external CD/DVD drive is good.
If so, then the problem lies within the B2. There is an option to reset the CD which is found from the front panel in 'Settings' --> 'Maintenance' --> 'Advanced' --> 'Reset CD' which might sort the issue out, so try that first.
Let us know how you get on.
Regards,
Peter.