We tried to do the ‘check’ you suggested, but the ‘Verify Backup’ was greyed out.
We also tried to erase the WD My Passport, with the idea of starting all over again, but were unable to erase it!
We tried several suggested ways of erasing the WD but to no avail.
As I said previously, the WD My Passport is a POS. If it isn't working, that isn't at all surprising. They have a horrendous failure rate. You should return it to WD under warranty.
I'm wondering if the problem is being caused by Ventura as prior to this the WD worked fine.
No, there is nothing wrong with Ventura that would cause such a problem. At least not that anyone has reported.
I've checked the WD software, which is up to date.
An external hard drive for a Macintosh shouldn't require any additional software. Which makes me wonder what "WD software" you are using, what it's supposed to do, and if maybe it is the cause of your problems.
Personally, I'd call WD support, and ask them to either guide me on making the hard drive work, ask them to replace it under warranty if they think that it is defective, and if they don't think that either is appropriate, ask for my money back.