i'm sorting through this problem now.
i picked up a lightly used G3 that looks great and has had zero issues so far. i took a samsung 128gb card out of my lenovo tablet and put it in the phone and tried copying music over to it.
it would copy nicely at first but eventually it would error and ask me to stop copying or skip that file it was having a hard time with. neither choice produced good results and i would have to unplug the phone and put it back in to keep copying.
the next day at work i went to listen to music and the player said "unable to play file" and called it out by name. tried a 2nd player, same thing. got home and took the card out to look at the music and while the folder was there, now the music was simply gone. several folders were empty. reformatted the card to try again via the pc.. overall i copied around 70+gb to the card and put it back in the phone. when copying to the card on the pc no issues. all copied fine and i could see the status bar fly along nicely. put it back in the phone and try copying, same copy error.
i eventually gave up at 70gb cause that's a lot of music and put it back in the pc, copied some final music and the pc said it was over 1/2 full. stuck it in the phone and it said 109gb free of 125gb. so somewhere from taking it out of the card reader on my pc and putting it in my phone it couldn't see the music anymore. unmount and remount the card, still 109gb free.
any idea why this is happening and what i'm doing to cause the music to be deleted?
1) not using google play.
2) i didn't unmount the card before i took it out but after out, it was reformatted on the pc, music copied over, and put back in the phone and most missing. have not put back in pc yet but will tonight.
3) standard usb cable to a powered usb hub to my pc is what i'm copying to the card with. class 10 samsung 128gb card.
4) cant copy to the card when in the phone but can just fine when on the pc.
5) unmounting and remounting the card doesn't change 109gb free.
getting a 64gb sandisc class 10 card and adapter to make sure nothing strange is happening but if anyone has any other ideas i'm open to suggestions.