USB Flash Formatting
In order to check whether your player is formatted correctly the device must be functional and able to communicate with your computer.
Connect your player then right click on your "My Computer" icon and select "Manage" from the menu ... from there, click on "Disk Management" and check the pane on the right hand side to see the format status of your player ...
You ideally want to see -> FAT and Healthy and Active as per example.
The fundamental basics are that before any kind of recordable media can hold formatting information it must first have at least one primary partition defined on it. A RAW drive or media cannot hold formatting information.
Specific to the S1mp3 player - Your computer may show your player as a USB device (see USB modes) in Device Manager (windows) but will not show up as a readable/writable device or drive in "My Computer" (windows) unless and until it is correctly partitioned AND formatted.
Although some players can be formatted with different file-system formats - it is recommended that for the Z80 on-board processor to work and (more importantly) for the music/audio decoding part of the firmware to function most efficiently - these players are best formatted to *FAT16 (aka *FAT).
* FAT = File Allocation Table
For S1mp3 player formatting tools see Software tools.