How to erase unaccessible external backup drives

That depends on why they are "unaccesable".

If they are simply not showing in Finder but do appear in Disk Utility, you can erase them from there by selecting the top level drive for them and then clicking on Erase.

click here ➜ Erase and reformat a storage device in Disk Utility on Mac - Apple Support

If they have failed and cannot mount and as such are not visible even in Disk Utility, then there's no way to erase them from a computer. But you can take a magnet and give it a few passes over the drive to scramble the data. Or take a hammer to the drive in question.

Inaccessible in what way?

Obviously there would be no way to erase an inaccessible drive.

Otherwise: Erase and reformat a storage device in Disk Utility on Mac - Apple Support

If the old drive's plug is not compatible with your new Mac, you might use a plug adapter, i.e. USB-A to USB-C plug adapter.

Did you install any third-party anti-virus/security software that by default blocks mounting external drives?