BaseCamp for Mac had ceased detecting my Drive 53 device. Express detected the device, but I made sure no vestige of Express was running simultaneous to BaseCamp. Cable and connections were fine. MacOS security settings for Basecamp were correct. I went through every step in the various Garmin support pages on this issue, all to no avail. So...
...Finally I went old school.* I backed up BaseCamp, deleted the BaseCamp app from the Mac, downloaded BaseCamp from Garmin, reinstalled it and restarted MacOS (that last step makes the difference!). Now BaseCamp detects my GPS device.
