I lost my 520 USB cable and tried using a different Micro USB cable. I can charge the device, but not see it mounted on my Mac. I have tried a couple of cables and no dice. I dont see an option of buying a cable from Garmin.
I've never even taken my Garmin supplied USB cable out of the box and just use one that I have lying around from and old Android phone because it's longer.
I don't know why you are having issues but just about any USB cable should work fine..
Many USB cables are for charging only. The cable for connecting the GE520 needs to be a data cable. Either you need to buy it that way, or keep trying cables till one works, then you know it's data capable.