External Hard Disk selection

Dear All,

My Western Digital external hard disk quit backing up. When I try to verify it, the Verify Disk Permissions are greyed out. When I try to find it with Time Machine, there is no disk visible when I get the "Choose Backup Disk" screen.

How do I choose my external hard disk again?

Thanks,
Steve

With it connected do you see it in System Preferences > Startup Disk is it is bootable? One reason why prefer SuperDuper.

And Time Machine is not bootable.

It shows my hard disk, then a second icon of what looks like the external drive with a big question mark on it.