Thank you for this amazing thread, with accurate instructions! I registered just to say BIG and FAT:
Thank you!
Updated my "beated by offroad use" ZUmo 590 to 595 with no issues and gave it a new life (transfering gpx routes from phone to Zumo is a must in offroading these days, since majority of my friends use KMZ and KML tracks format, which now I can convert on the fly on smartphone AND send promptly to mu Garmin. All that in 5min on the track itself!!). Thanks again!!
Another thing -
I can confirm that I did installation using virtual machine: Parallels VM + old WinXP VM. Trick is to dedicate usb connection to VM and to have non generic usb drivers (so updater can recognize Garmin device).
Anyway, it is possible and you do not need actual Windows box to do the upgrade!
Cheers from Belgrade!