Thanks for the replies. Yeah, same problem regardless of boot connected or boot then connect. Same problem regardless of which ports or which USB cable. My other two SATA drives are WD, so the drivers should be OK. Anyway, I used Paragon Partition Manager and finally managed to assign a drive letter to the USB HD. That solved the problem. But, it is still a mystery why Windows wouldn't assign a drive letter on boot or on connect.
Thanks and regards, Jim