How do I erase an external HDD on my MacBook Pro?

use this article:

Initialize a external storage device in Disk Utility on Mac - Apple Support

it shows extra selection to "show all devices".

You what the erase the Physical DEVICE, by its immutable manufacturer-given device-name.

If it says no, just pretend it said "are you sure" and immediately ask again.

There is an Unmount button in the header near the erase button.

what happens when you select the drive and click unmount?

if it tells you it can not unmount, restart in safe mode and try again.

Glad you stuck with it and won out over the machine.