The MP3 player is a USB storage device
The most common mass storage devices are flash drives, memory sticks, and jump drives.
The MP3 player is using USB 2.0 or USB 1.1 connection
The MP3 player is of the correct format
Supported format are FAT12, FAT16, FAT32 (sector size: 512 - 65,536 bytes).
The battery level of the MP3 player is adequate
Make sure that the player battery is charged or the battery is fresh.
Note: In some USB flash players (or memory devices), the stored contents are recorded using copyright protection technology. Such protected contents will be unplayable on any other devices.