Let us go on a technical story and explain a key simple fact I overlooked.
What you see below is the Terminal app included in Mac OS X. I resorted to using this app because I have a 4TB Seagate External Hard Drive that wasn't being recognized when I am connecting it to the Trash Can Pro (Late 2013 Mac Pro.) So I went to Google and did my normal search.
Skip to the second section if you are looking for ideas of how to recognize the disk.
An option was to use the diskutil command line, which I did. I was seeing the hard drive but whenever I tried repairVolume or repairDisk nothing was being fixed. Then I found a KB article on Seagate saying to make sure the AC adaptor is plugged into the actual wall, not a power strip. I kept pulling my hair and then tried eraseDisk as seen below.
On a random whim, I thought to change out the USB 3 cable and see if that was the issue. Son of a bitch, that was the culprit preventing this drive from showing up in Disk Utility or Finder. Swap cables our first if you have issues with your external hard drive on Mac.
If you continue to have problems, then take/raise the appropriate levels of force gradually. No hammers kids.
Section 2 - Ideas on how to fix.
Open Terminal - CMD Spacebar and type Terminal or go to Utilities under the Applications folder and then select Terminal.
Using diskutil you can use other commands to work on the disks like:
example: diskutil repairVolume /dev/disk3s2
repairdisk - A whole disk must be selected to repair a disk, not just a partition.
example: diskutil repairVDisk /dev/disk3
eraseDisk JHFS+ (Note, you will need to identify the file system e.g. JHFS+ and then list the new name of the hard drive)
example: diskutil eraseDisk JHFS+ Audi S8 /dev/disks3s2
I hope this helps. This is how far I have made it in Terminal researching the internet and trying out commands.
You can also check out this article - Disk Management From the Command Line