error 50

I have a 3TB WD external HD hooked up to my new iMac 2015, running El Capitan. The HD behaved just fine on my dead Mac Pro 2009, however when I right click New Folder or File/New Folder I get "The operation can't be completed error 50".

I found one thread suggesting that "Put hard disks to sleep when possible" under System Preferences/Energy saver does the trick, doesn't work for me. All other save external HD's work just fine.

Any ideas?

Thanks

Mark