Yes this is the default behavior, you cannot use the USB while rockbox is booted.
If you want to plug in your MP3 player without having it boot into Disk Mode, then hold menu while plugging in your USB cable, now rockbox will only charge

Understand that oou wont be able to browse the files on your mac/PC while using this method.
As for having to manually reset, mine used to reset automatically but different people may have different experiences