I plugged my Kindle into my Macbook. The Kindle went to “USB Drive Mode” and locked up. The Mac did not recognize it as a USB drive. (The Mac ignored it.) The Kindle did not leave USB mode when unplugged. I’m using OS X El Capitan 10.11.6.
I had to hard-reboot the Kindle by holding in the power button until it restarted. The problem was 100% repeatable.
After several tries with my Macbook, I rebooted the Kindle and plugged it into a Windows notebook and it worked as designed. So it was a problem with my Macbook.
I unplugged everything from my Macbook, rebooted the Kindle, rebooted the Macbook, and re-connected the Kindle to the Mac. It works. 100% repeatable.
I suspect that VMware Fusion had messed up my USB port. (I exited VMware, but that didn’t cure the problem. I had to reboot the Mac to fix the issue.)