What OS do you have on the Mac? (El Capitan did the same thing to me until, I think, patch #3, or a later Garmin update fixed it - I updated both at the same time so I'm not sure which actually fixed it, they both blamed each other!) . Mac forums seems to be full of similar 'drive not recognised' issues throughout the ages (although Garmin hardly have a flawless record themselves)..
What OS do you have on the Mac? (El Capitan did the same thing to me until, I think, patch #3, or a later Garmin update fixed it - I updated both at the same time so I'm not sure which actually fixed it, they both blamed each other!) . Mac forums seems to be full of similar 'drive not recognised' issues throughout the ages (although Garmin hardly have a flawless record themselves)..