This just started recently (within last month or so).
Catalina 10.15.6
System Preferences / Security & Privacy / Privacy tab / Files and Folders / Garmin BaseCamp.app - Removable Volumes IS CHECKED
I have thoroughly removed and reinstalled BaseCamp, including tossing all data
Garmin Express sees my device and recently let me update to the latest map and firmware version - 4.6.0
Garmin Maps - Full Coverage of North America 2021.20 (on both device and hard disk)
NOTE - after updating to the latest map two or three weeks ago it still worked
Conclusion - recent Catalina update to 10.15.16 OR recent (I believe) update to Firmware 4.6.0 caused this issue
Please help!
One final comment. I have been using BaseCamp for about a year and a half now. Given the number of issues I've had, all which (so far) have eventually been resolved, I can come to no conclusion other than Garmin doesn't really care. If they did they would test their software THOROUGHLY against OS updates prior to those updates hitting the street.