External hard drive not showing up

Have you checked Disk Utility to see if the SD partition is actually showing up along with how much space is used (would need to be at least the space used for your internal)

Launch/Utilities/Terminal and copy & paste this command at the prompt:

diskutil list

Press return.

If it sees the disk, note the number of the disk (far left column) you want to mount.

Then, copy & paste this command:

mount /dev/(number of disk)

Put in the number no parenthesis.

Press Return.

Also might try

mount force /dev/(number of disk)

What format is that Drive?

Did you do an OS upgrade lately?

EtreCheck is a simple little app to display the important details of your system configuration and allow you to copy that information to the Clipboard. It is meant to be used with Apple Support Communities to help people help you with your Mac.

http://www.etresoft.com/etrecheck

Pastebin is a good place to paste the whole report...

https://pastebin.com/

Or use the paperclip at the bottom of a Reply to attach the full report here. :)

Workable but harder for me to work with...the Note tool on the bottom of this editor's toolbar, as shown in the image, to copy and paste the output from EtreCheck.

It should not, but are these external drives USB Bus powered?

In Sys Prefs>Energy Saver>Put Disks to sleep when possible checked?

Check System Report>HardWare>USB for mA used..

It doesn't sound like you made a clone, just that you opened SuperDuper to see if it would recognize your drive that way?

The right way to navigate your TM backup would be to go to Photos folder in Finder then launch TM from your Dock.

FYI, a clone (using SuperDuper) should be on its own space/partition. So, when you set up the external drive, did you partition it? One for TM and one for the clone? If not, then there is no SuperDuper backup.

Was theSuper-Duper a clone?

If so hold alt key on startup to choose the boot volume.

For TM, in Finder negotiate to Photos folder, then Enter TM.

SuperDuper! is a cloning app, basically a cloning app, the other popular one is Carbon Copy Cloner. In short a cloning app makes an identical copy of the computers primary (normally internal) drive which can be used as a backup and in the event if the internal drive dies (they all do eventually) allows you to start and run from the clone until the internal drive is replaced.

It is a VERY VERY bad practice to put your clone and Time Machine backups on the same EHD, why because all HD's eventually die and when the EHD that holds the clone and TM backups dies you have lost ALL of your backup. Don't laugh, it happens all the time. And it usually happens at the worst possible time like when your internal drive decides to fail which leaves you completely dead with nothing to restore.

Many experienced users believe in redundant backups, using TM on one EHD and another EHD for a clone. Some backup their clone 1x per week and then keep the clone offsite in order to protect against fire or theft. Others just run both backups simultaneously which is what I (possibly foolishly) do.

Buy a dedicated EHD to use for SuperDuper! They are inexpensive so there really is no reason not to!