I may be wrong but isn't 32gb the biggest card you can use ?
There is contrary advice on the internet.
BMW's own website seems to indicate 64gb, whilst Garmin's own site recommends 8gb. Meanwhile, there are reports on other websites (including UKGSer) of people using cards up to 32gb.
The best overall advice seems to be:
1. If you just want to run the maps and nothing else, 8gb is sufficient. I do this; it works, giving me enough space for the latest mapsets for the whole of Garmin's Europe maps and large chunks of Germany / France in Open Street Maps. More than sufficient for my needs as I do not use any music.
2. If you want more space, 32gb is the largest card to use.
In both instances, make sure the card is correctly formatted before use.
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As regards the rest of large chunks of this thread, I've just installed the latest maps to:
(A) My Mac and a Nav V simultaneously without a glitch, using the install to computer and device simultaneously option within Garmin Express. It apparently took six hours, so I just left it running overnight. However long it took, it was all done when I woke up.
(B) To a second Nav V only, again using Garmin Express. I didn't need to install them to my Mac, as they are already there. The process was estimated at taking 30 minutes. I didn't bother to watch but the maps are safely installed.
I shall be installing the same maps to a third Nav V and to a MacBook Air this evening, again using Garmin Express. Assuming that goes well, that will be three Navigator V's and two Mac computers updated with the latest maps, all without a glitch.
There are now so many secondhand Nav V devices floating around, many of which have at sometime in their lives been attached to PC's or Mac's and / or been used with different SD cards and / or been 'improved' by careful owners, it is all but impossible to tell why bods might be having problems. Add in that it is often impossible to tell quite how some people have their PC's and Mac's configured and what Garmin software (Garmin Express / Garmin Mapinstaller / Garmin Mapmanager) they might have installed or be using, it's getting very difficult to help them.
All I can say is that I have rarely if ever had a problem using my Garmin GPS devices, whether in the past on PC's or more recently using Mac's. I am far from an IT whiz, having no idea how to do anything more than just limiting myself to using Garmin software, attaching the devices and patiently following the instructions on how to proceed. If it takes two minutes or ten hours, I don't care. I just leave it to do its stuff. I use the most basic SD cards, no larger in size than is absolutely necessary.
If for some odd reason a card does not work just as it should or if Garmin's software for some unknown reason prompts me to change a card, I just do it. Cards are cheap; querying why a card that should work but suddenly doesn't is not worth the effort. Swap in a fresh card and normal service is resumed, though this happens so rarely I can't recall when I last had to do it.
Keeping things simple seems to work for me.