I have upgraded to a new (2024) Mac with M3 processor, running IOS Sonoma 14.2.1. I have two Garmin satnavs, a Zumo 345 and a BMW Nav6, and although these would show and update in Garmin Express they would not appear as attached devices in Basecamp. I found a download for Basecamp v 4.8.13 and installed that but nothing improved.
Searching here and all over the web said that it was do do with 'privacy, security, files' but I could not work out what was meant. I have now found that if you go to the Apple Logo (top left)/System Settings//Privacy & Security/FIles & Folders/ then you will see the Garmin Basecamp and Express app logos and you need to click in there to give them permission to access external devices. Express already had the permission, but Basecamp was set not to. A simple slide of button!
Other web comments say that if Express is running then Basecamp will not find a device (satnav). Well I disagree, as I can now have Express looking at a device and Basecamp finds it very quickly. Everything is now much faster than it was on my 13 year old Mac.
Norm.