excellent rapid troubleshooting response
Amazing! I followed your Acasis advice. After powering off my Mac Studio and disconnecting my TB501Pro for one minute. I held down the power button of the Mac Studio for 15 seconds, entered my password and WD4T icon (WD850X) appeared 1st time in a week!!!! I am now using the Apple TB5 expensive cable to get the best transfer speed and I also removed my OWC external backup disk (several years old ) to simplify troubleshooting. Thanks. I expect TB501Pro failure to be on Disk utility list no longer appears. Black Magic 5691.2write\5907.4read.