I have a new MacBook Pro, an external USB-C HDD for backup, and an Android portable music player, also with USB-C. I got a cable to connect the HDD to the MBP,

I have a new MacBook Pro, an external USB-C HDD for backup, and an Android portable music player, also with USB-C. I got a cable to connect the HDD to the MBP, and it works fine for that purpose, but when I use that cable to connect the PMP to the MBP, it doesn't work: the MBP doesn't mount the PMP's filesystem. But if I use a C-male-to-A-female adapter, then an A-to-C cable, the PMP mounts just fine. Anyone know why?