Add file on USB key. But loose all

Hi,

On Pi, PcP and LMS an USB is connected with some music files.

It works.

Now remove the USB key, add some music files and replug into the Pi.
On LMS, settingss rescan, browse to my folder.

But nothing. All was gone. But the files are already on the USBkey,
I probably did something wrong when I added files.
But what ?

Same behavior on FAT or APFS. (Second question is better to use FAT or APFS???)

I'm not clear on waht is the "mount point". I set the same name as the label.


Thanks

PS