How to remount external drive in OS X El Capitan without rebooting? I have a MacBook Pro 15 Mid 2014 with OS X El Capitan. 
When I accidentally disconnect the external USB disk (without clicking eject), I can't reconnect the disk. If I remove and plug the disk back in it doesn't appear in the Disk Utility.

The solution is to reboot the system.

Is there some command to reconnect the disk without rebooting the system?
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.