Disk Unable to Mount
Try clicking on the red highlighted parent disk and format that as OS X Journaled. Note that formatting the disk will erase all the data. So if you wish to keep the data and use the drive normally then please comment below.



NOTE: The following method will recover your data but all of it's name will not be the same you will lose the names of all the files if you do not want this to happen the comment below and follow the steps very carefully. As even one wrong step could damage you hard disk permanently and do not quit the terminal window directly after finishing recovery always exit the program in a grace manner like choosing the 'Quit' option. 

As you told you want the data on the disk then first download testdisk for Mac first.Make sure you  connect your damaged external disk in which you wish to recover data from

Then extract the zip file then search in spotlight for "Terminal" then type in cd ~/Downloads/testdisk-7.1-WIP. Then run ./photorec and choose your correct hard disk which is damaged. (Just don't choose 'rdisk'):



Then choose the disk again don't choose 'rdisk' then choose the disk with HFS don't choose 'No partition' then if you wish you exclude some files then choose the 'File opt':



Then click on "Search" then click on "Other". Then after that choose the option "Whole". If you face any trouble please comment below! Quit the terminal window directly after finishing recovery always exit the program in a grace manner like choosing the 'Quit' option.