Why won't my late 2013 Mac Pro running Yosemite won't burn DVD/CDs to an external drive?

My Mac Pro won't even mount the external drives (SuperDrive & Pioneer BDR-XU03), even though mounting is ticked under finder>preferences. It will allow me to erase, but not burn. If I choose "Burn" nothing is highlighted anywhere for me to choose.

I'm running Yosemite 10.10.4, all software is updated. The Pioneer was burning intermittently the last time I used it. Basically I'm trying to burn data DVDs.

Processor: 3.5 GHz 6-Core Intel Xeon E5

Memory: 32 GB 1866 MHz DDR3 ECC

Storage: 1 TB Flash

Thank you.