Hi and thanks for your submission.
Do you know if it will appear in Disk Utility?
In the Finder, click on "Go" in the menubar and choose "Utilities"
Locate the "Disk Utility" app and open it
In the left hand sidebar, choose your external Hard Drive, if visible.
On the right hand side of the page, click "Verify Disk" (If the procedure uncovers errors, please click "Repair Disk" to repair)
Once the process has completed, restart your Mac.
Now see if the drive appears normally.
To clarify, were you able to verify the disk without error?
If so, see if the drive appears in Safe Boot mode.
Restart your Mac
As soon as you hear the Startup chime, press and hold the SHIFT key
Continue to hold the Shift key until you see the Apple logo and progress indicator.
Then, let go of the Shift key and let it boot to the Desktop (it may take longer than usual so please be patient).
Now test for the drive.
Next restart and let me know the results.
When in Disk Utility and you select the volume, do you see the normal stats at the bottom of the window indicating drive space used, etc...?