Many thanks for your prompt replies. The logic behind my question is that the QRH for the SOURCE OFF light - together with the GEN OFF light - directs you to attempt a reset first - provided that there is no indication of a short such as smoke or fumes or bus failure but i am diverging now! - while the DRIVE nnc directs you to manually disconnect the idg, which is irreversible in flight, without attempting a reset - which makes sense cos anyway the automation has already disconnected the CSD from the agb, if i am not mistaken. One instructor once told me that if you have the DRIVE light on to carry out the DRIVE checklist first to make sure that you isolate the CSD right away cos if you try to reset the generator the drive portion might overheat and fuse together with the rest of the components of the IDG making it impossible to disconnect it and thus causing a high level of vibrations in the engine - war story that i am just reporting and that according to him took place in a DC9. I was just thinking or maybe overthinking what could make the generating portion of the IDG to fail and the CSD portion still working.
anyway thanks a bunch