Garmin's help article recommends allowing their support personnel to remote into your computer to [presumably, from the description of the workaround] issue a sudo command to do a permissions change--YIKES! If you are going to go this route, PLEASE be aware of the risks and take steps to ensure it's actually Garmin you're talking to on the other end of the phone!! Have them clearly tell you when they are beginning and when they are done, and CHANGE your computer's Administrator password after they've finished. Make sure that whatever they ask you to do to grant them remote access (write each step down), they also have you un-do after they're done. For every step they have you take, they should be able to point to a step later that un-does it.
I don't want to be Debbie Downer or dissuade people who aren't comfortable with doing this change themselves. I'm also not intending to disparage Garmin. Their tech support is outstanding and they are clearly bending over backwards to help with a difficult-to-solve problem. But "remoting in" to your computer is the same method those "tech support" phone scammers (which you hear about targeting the elderly) use in order to do their delightful business. So, PLEASE double check the phone number you are calling and make sure you're actually talking to Garmin.
(techo tangent)
Interesting workaround though, changing the UNIX permissions on the raw disk character device. This would imply database updates need to do much deeper work on the SD card than simply copying files to an existing volume. The vast majority of applications out there that load files onto an SD card do not need this kind of access. I'm a Mac developer and the only reason I'd want to access /dev/disk* or /dev/rdisk* is if I was loading a full, pre-built image onto the device. I don't want to Monday Morning Quarterback their software, so best of luck to them as they work on an alternative.
(end technical rant)