Hi
Does the drive appear in Windows Explorer?
If not...
Go to Disk Managment and see if the drive appears there?
If you see the drive in Disk Management, make sure that it has a letter assigned to it.
I don't recommend using the native backup app, I would suggest EaseUS Todo Backup.
I've used it for years, it's the easiest backup software to use, literally once you have selected a location you can make a backup with 3 mouse clicks.
And best of all it's free!
Download free data backup software - EaseUS Todo Backup Free It has never failed to recover my PC, I had to do it at least 7 times when beta testing Windows 10 and several times since.
It may be that just using different software will fix it if it's showing up elsewhere.
If you still don't get the drive to show in the backup program but it shows in Windows Explorer, then try formatting it again.
If that doesn't work let us know, and there are more advanced steps to take.
Also if it's not showing up at all, try plugging it into another computer.
Mike