MP3's missing from MicroSD Card in Galaxy S4

[NzCam]

I'm having trouble with music stored on my Samsung-brand 64gb MicroSD Card.

After copying DRM-free .mp3 files across, anything more than roughly 8gb worth in total disappears. The files that go missing are always the most recently added;- so anything under that 8gb threshold seems to stay on the card, but beyond that, I'm left with an apparently blank file folder.

This happens with music copied via USB cable and via card-reader. If I disconnect the SD-card and insert it into my laptop, I can copy a full collection, 20-30+ gig worth, and manual inspection / playing random songs confirms that they were successfully transferred to the SD card - so it's not the SD card on its own. But when I re-insert the SD card into my Galaxy S4-i9505 (JellyBean 4.3), bingo, the most recently added songs have disappeared and I'm left with the blank folder again. If I put the SD card back in my laptop, those songs are still missing or deleted or whatever is happening to them.

The available space on the card is shown as 54.97gb of 62.50gb, but the total amount of music transferred I know to be 12gb or more depending on what I've tested. I've tried formating as exFAT multiple times, both by the S4, and by Windows on my laptop. I've also tried NTFS format, but the S4 can't read that.

What's the cause? Can it be overcome?

 

[NzCam]

Update on issue: a Samsung customer service representative suggested backing up the device then doing a factory reset. I felt this was a long shot, but did it just to rule it out. The problem still occurs even on a "factory" device with no settings changed or new apps introduced. The 8gb safe/more gets deleted thing is too consistent to be coincidence. Are S4 GT-i9505's incapable of handling more than 8gb external ROM, inspite of what the stated specs say? Do I need to do something tricky with a partition?

Someone please drop a knowledge bomb on me!? :-\