I installed El Capitan some time ago, and today for the 1st time since then decided to use my external CD-drive. The latter worked perfectly with previous versions of OSX, but EC does not recognise it. I have tried to find and mount the drive in Disk Utility, but without success. In one of the discussion-groups, I have seen this suggested as a swift cure for the malady: 1. Launch Terminal. 2. Type: sudo nvram boot-args="mbasd=1" 3. Enter Administrator-password, press Return. 4. Reboot. Is that a safe course of action ?