To those with the 2016 MacBook, you may want to hold off installing the latest El Capitan Beta if you need to access USB drives. I noticed that any drives requiring USB 3.0 power do not seem to mount with the new beta. If I revert to 10.11.4 the drives mount with no issue. Kudos to the Genius Bar rep who spend 30 minutes with my MacBook trying various tests and then encouraged me to try downgrading before sending it back (saved me the hassle of being without the MacBook for a few days).