MicroSD with vanishing files.

SanDisk MP3 players Sansa® Clip | Sansa® Clip+
11

I just remembered–there is a quirk with FLAC files. On the external card, they need to be inside a folder called Music. 

Don’t ask me why. One of those things. 

http://forums.sandisk.com/t5/Sansa-Clip-Sansa-Clip/FLAC-files-ok-internal-memory-not-on-the-micro-SDHC/m-p/316689/highlight/true#M45873

On the other hand, my external card has a whole lot of mp3 albums in the root directory with no problems, so that doesn’t explain your mp3 difficulties. 

And the absence of tags in FLAC means you’ll always have to use Folder navigation.