External WD "My Book for Mac" changed to Read Only

I suspect the HD has failed. As a general rule of thumb WD EHDs are pretty much throw away drives as they do not supply adequate cooling, meaning the enclosures are sealed and cooling air cannot get in which leads to premature failures. Why does WD make them this way, simple they are cheap to make and sell and mean (some) customers will come back and buy more.

The solution is to buy high quality HD enclosures such as the OWC Mercury Elite Pro series. These are a little more expensive but last MUCH longer. When the HD eventually fail (they all do) with a simple screwdriver you can open the enclosure and inexpensively replace the internal HD.

To verify my hunch about the WD drive failing, download DriveDX and test the WD drive. If ANY errors appear that simply confirms my hunch.

All drives including external drives have an undetermined life span and can fail at any time.

Disconnect the WD from your Mac.

Pickup another external drive and start new Time Machine backups on it.

Then go back to the WD and see if you can Repair it with Disk Utility or optionally Erase and use it for redundant backups.