Not for the faint-hearted, so don’t do it if you’re nervous. If you ignore this warning at least make a decent back-up before you start. Background: Here’s how it started (for me anyway). I replaced my existing hard drive in my MacBook Pro with an SSD Drive from Crucial (a BX200). I also bought an SSD/Hard drive caddy and an external Superdrive enclosure from eBay in the UK (seller thenatural_2020 (3135)) - I believe OWC do these if you’re in the US. I put the SSD in place of the original hard drive, and put the original hard drive in the optical drive bay using the SSD/Hard Drive caddy. The optical drive I removed was installed in the USB drive enclosure ready for the rare occasions that it still gets used. After the usual faffing about cloning my old hard drive and booting up with the new SSD I tried a dvd in the optical drive. All I got was an error message - A valid DVD drive could not be found. [Error -70012]. Note to Apple software engineers - if you have to put an error dialogue in your software please make it at least partially understandable, not this sort of gibberish. Sorting it out: I claim no credit for the following, and have provided the links rather than rewrite all the original. Firstly, the reason DVD Player won’t work is because it is looking for an internal drive and not an external one. The fix is not trivial (ie you can’t do it by ticking a box in a preference pane somewhere) and is made more complex by Apple’s use of System Integrity Protection to prevent even root users from modifying system files. so, step 1 is to locate the relevant DVD file and change it to reference an external drive. Go to ~/System/Library/Frameworks/DVDPlayback.framework/Versions/A to find the file called DVDPlayback. Make two copies of it, one as a backup and one to modify, and stick them somewhere so you can find them again like your desktop. Now for the nail-biting bit - turn off the System Integrity Protection. Follow the instructions on http://apple.stackexchange.com/ques...ity-protection-sip-aka-rootless-on-os-x-10-11, which involves a reboot into recovery mode, starting Terminal from the Utilities menu and typing <csrutil disable> followed by Return. This will effectively allow you to change and modify the file (something that even root can’t do in El Capitan). Re-boot normally and you’re ready to modify the file. Reboot your computer. To modify the DVDPlayer file - go to http://www.cnet.com/news/addressing-dvd-player-error-70012-when-using-external-drives-in-os-x/ for the full instructions. You need to download a hex editor (eg 0xED) and use it to open one of the copies of DVDPlayer on your desktop. Use the menu to Find.. the text “Internal” and replace it with “External” - there should be 4 instances of that word in the file. Save the file, and then drag it from your desktop into its’ proper location, ie ~/System/Library/Frameworks/DVDPlayback.framework/Versions/A. You’ll be asked for your administrators password to allow the new file to replace the old one. The final step is to reboot, and test your DVD Player using a DVD, not a data disc. If all is well, reboot again into recovery mode (see above) and use Terminal to switch SIP back on again, by typing <csrutil enable> followed by Return. Reboot once again and if you want use Terminal again to check SIP is active by typing <csrutil status> If it doesn’t work I can’t help you - it worked for me on a mid-2012 Macbook Pro running OS X 10.11 with an Apple Superdrive as the external drive. Some other suggestions for fixing the problem that I came across were Repair permissions. Really? I’ve never known this to fix any problem whatsoever, and in any case it can’t be done in El Capitan unless you use Terminal or maybe Onyx. Use VLC. Well, ok, but it’s not really a fix is it. A kludge maybe but certainly not a fix. Using a script called DVDDriveSwitcher - I couldn’t locate this script and in any case all the references I read related to much older versions of OS X, so it might not work on El Capitan. Use Disk Utility to repair the drive. I don’t think so. Re-boot the computer. Hahaha. Buy a new external optical drive. Like I’m made of money. Many thanks to all the geeks out there on the internet who put up all the info I needed to sort this out.