yesterday, I got my SanDisk 400GB miscroSDcard.
I copied 335GB of Music on the PC and then inserted it in Slot 1.
It works in FiiO X5 3rd without an issue ...
Will play a few days with it and report, when something unexpected shows up ...
Edit:
After some more time to play with, I need to add, that there are issues, when you're not patient enough. My standard way to play music is with folders. I have around 5'000 albums and play them usually in the folder mode. That works like a charm ...
But, if you like to shuffle or search for a specific title, you're in trouble. I guess, the processor of FiiO X5 is simple not performant enough to do this tasks in a way, that it is useful. The initial scan in FiiO Player took about 10 minutes, PowerAmp roughly half an hour.
So, if you want to use seamlessly all functions when playing music, the 400GB microSDcard is not the right storage solution.
For me it works, I can go to an artist folder, chose the album and play it ... That's how I use FiiO X5 ...
Some ScreenShots you can find
here