[PROBLEM] Moode won't start if a USB stick with music is present

[Ouchita]

I second Al's suspicion that the USB stick with music is marked as bootable. (As always, one has to keep in mind that if the power supply is inadequate, then all sorts of weird behaviors may occur but it sounds like this isn't the case here. One can always check the logs for undervoltage events.)

I routinely run test scenarios, including on a Pi3B+, which boot moOde from one USB stick (actually a uSD card on an USB adapter) and contain test tracks on one or more additional USB sticks. This approach has been working for me since the RPF introduced the ability to boot from USB.

Regards,
Kent

Thanks to both of you. I use 64 Gb Sandisk  Cruzer sticks, NTFS format.
I wasn't aware that a USB stick can be marked as unbootable, despite quite a few creations of loading music on an empty stick.  How do I achieve that?