I accidentally unmounted the External Hard Drive and now I want to re-mount it back So I had the external 4TB Seagate Expansion Drive, and at one point I end up selecting unmount in Disk Utility when I tried running first aid. 

Now I wish to mount it again, I tried pressing the mount button in Disk Utility. No response. 

I tried ejecting, unmounting, and re-mounting the backup drive in Terminal through: 


diskutil eject /dev/disk3s2
diskutil unmountDisk /dev/disk3s2
diskutil mount /dev/disk3s2


The resulting response is:


Volume timed out while waiting to eject
Unmount of all volumes on disk3 was successful
Volume on disk3s2 timed out waiting to mount


Insofar the hardware is blinking so I guess it's functional. I just like to ask if there are any solutions possible, or should I just click the mount button and wait for it, thank you.

Bonus: Here is the Disk Utility for the external drive insofar
There is a 3rd party app named Mountain that lets you control volumes on you mac. The app has a free trial period and costs around $6.  Also available on App Store.