While booted from your internal hard drive, launch Disk Utility. Select the external drive (the physical drive, not any subsequent partitions) and click the Erase tab. That will give you options for formatting, so choose one and then click Erase.
Jon
macOS 11.7.10, iMac Retina 5K 27-inch, late 2014, 3.5 GHz Intel Core i5, 1 TB fusion drive, 16 GB RAM, Epson SureColor P600, Photoshop CC, Lightroom CC, MS Office 365
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