Your computer uses USB Mass Storage Mode to detect your Garmin device. If your computer does not detect your Garmin device, you can try these troubleshooting steps:
1) Unplug the USB cable from the computer
2) Turn the unit off and then back on again
3) Plug the USB cable into your computer
The device should automatically go into USB Mass Storage Mode. USB Mass Storage Mode is indicated by a small picture of a device connected to a computer tower on your device's screen. You may check the computer's recognition of the device on a Windows-based PC by clicking on your Start menu and selecting My Computer or Computer. The device will show in this window if the computer recognizes it. On a Mac, the device will show as an icon on the desktop entitled "GARMIN" or it can by found by selecting Finder from the dock and viewing the device list on the left-hand side menu within the window.
Note: If your computer still does not detect the device, make sure that you plug the USB cable directly into the computer USB port and not a USB hub. If the cable is already plugged directly into the computer, try plugging it into a different port. If the problem persists, try another computer, if available.
Source: Garmin Official Site