How to erase and reformat my old Western Digital 1TB HDD for M3 iMac

See if you can see the WD drive in Disk Utility with the option to show all devices:

Then erase the container with the format either APFS or OS X Extended (journaled) with GUID partition.

If you installed any of the WD software that comes with their drives uninstall then according to the developer's instructions. It's not needed and can interfere with the systems file system management.

Have a look at > Erase and reformat a storage device in Disk Utility on Mac - Apple Support

I personally use some older external drives for redundant backups, but I would never use an old external drive for my main or only backup.

Plug the WD drive in to the USB port that you will be using, I’m assuming it’s USB A but some use Usb C to A adapter, Open the disk utility app, Click on the disk, Go to erase, Select APFS, give it the name you want. And select what type of erase security you want done. There’s 4 total options. Level 1-4. The higher the level the longer it will take to erase.

i have a WD passport I just erased using an M series Mac and if you use an adapter and erase it from a thunderbolt 4 port it will speed up the erase process on higher level/more secure disk clean ups.

Why not simply replace the drive?? The drive is very old and is likely very prone to failure. You can buy a high quality 2TB External Hard Disk for $139, please click OWC Mercury Elite Pro

You can't really show it as Answered anymore, just post the solution & ignore it after that. :)