format an old external hard disk
One of my old Macs (from 2010-2011 something) runs High Sierra 10.13.6.
It has an external hard disk used for Time Machine backups. This seems to have broken down. "Copying failed." The disk appears on the desktop and can be opened in Finder but that's as far as I get.
Used Disk utility to verify and repair the disk. Repair failed.
The buttons on top in the Disk utility window (erase, partition etcetera) are all greyed. out.
The files on it cannot be moved to the trash.
How do I re-format this?
iMac 27″, macOS 10.13