How to remount external drive in OS X El Capitan without rebooting?
You can, in Terminal, try:

sudo diskutil list


Check the output for any drive matching the external drive you have connected. /dev/disk0 is your internal drive. If you have no other drives connected, then your external disk should show up as /dev/disk1.

If you can see the drive, run, from Terminal (assuming your drive is /dev/disk1):

diskutil mountDisk /dev/disk1


This should properly mount your external drive.