Okay, was updating the map on my Zumo 595LM yesterday, and the unit shut off halfway through, due to low battery. I've charged the battery, and now get the above error message when connecting the unit to my Mac. Have tried different cables, different ports on the Mac, removed and reset the battery, and also done a factory reset on the unit, all to no avail.
After the factory reset, I was able to pull up three routes that I transferred out of BaseCamp yesterday, and it did import them, so that was good, but plugging the unit into the Mac again after that work, brought the same error message up. I ran First Aid in the Mac Disk Utility on both the Garmin zumo System Media and the Garmin zumo Card Media, and AFAIK, nothing was "Fixed".
Two things that might be worth mentioning...
The Garmin zumo System Media is empty and shows as not Initialized:
I can Initialize, (but haven't), by selecting Restore. If I do that, don't know what to select for the Restore From:
I assume it's the Garmin zumo Card Media, but don't want to blow anything up.
Second thing I noticed was that the Garmin zumo Card Media is full. 
Was this a result of the crash yesterday during the map update, and if so, is this the reason the disk is unreadable? I would have thought the factory reset would have wiped most of this data off, but it didn't do anything.
Thanks for any help!