This may not be an issue for the newer trucks, but my 2014 will not read anything over 128gb, I believe. I use an SD card, but I was limited to what capacity of card I could use.
The other odd thing about it... I have something like 5k songs (don't know the exact number) loaded on it, but my screen will only show 2500 give or take.
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When I had my 2015 with the SD card slot I was told by UConnect that 32GB was the largest memory, USB or SD card, that the system could accept but I've read many, many reports like yours of people using 64GB and 128GB SD cards and sticks.
For a while I was having trouble with the system responding to voice requests for songs, albums, artists, etc.
Me: "Play song 'Bad Company'."
Truck: "I'm sorry, I'm still indexing your library."
20 minutes of driving later, same thing--"I'm sorry, I'm still indexing your library."
A UConnect tech told me my 32GB stick needed to be subdivided into subfolders with 4GB maximum capacity. Funny, but my wife's 2016 Charger had no problem with any USB stick or SD card I threw at it. So I went to all the trouble to subdivide my USB stick into 4GB subfolders but the problem persisted.
Even though my system said I had all updated software patches I took it to the dealer and they updated it. Worked perfectly from them on. Bonus--I got rid of the cheerily smarmy female voice that came with my 2015 and got the somewhat sultry female voice of my wife's 2016 Charger!
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