Why can't the unit be charged with the USB cable supplied? It always says "cannot be charged with this cable". Do you have to order another one for charging on the Mac?
the problem with the message "The device cannot be charged with the cable connected" is described in detail at Garmin Support under the following link:
https://support.garmin.com/faqSearch...
I don't know why it doesn't work, but it also doesn't work via a Windows PC or via the power socket with the USB cable. It works via the car connection. Or I still have an old charging cable for the nüvi, which also works.