Yes, the Garmin drivers are to support older devices that do not present themselves as generic USB Mass Storage device.
Mapsource will 'see' these devices.
More modern devices will present as generic USB drives, and don't require Garmin drivers.
Mapsource will see these too.
So to troubleshoot the issue...
Is the device going into Mass Storage mode? ( The screen will show this )
If not, it possibly hasn't detected that it's plugged into a PC, just external USB power.
Could be a problem with the cable or the USB port.
Try a different cable.
Does the same cable / USB port work with any other USB device?