Just got a brand new, already formatted for mac WD 2TB External Hard Drive. When I plugged it into my Macbook Pro (mid-2010) using El Capitan, the drive was recognized- but when I click and drag it will not let me put files into it. It gives me error messages. STRANGELY when I plugged it into my older 2009 iMac running Snow Leopard, it works fine. On the box, it says it's compatible with Mountain Lion and up, so I don't even know why its working on SL vs my newer El Capitan. I already emailed tech support and they haven't gotten back. Any ideas?