There are several songs that are on the m240 that are not showing when you look at it's files - how are these hidden?

If you added songs to the device in MSC mode, then switch to MTP mode, you will not be able to see the songs added in MSC mode and vice versa, though the space used on the player will still reflect their presence. Switching modes should make the files show up. For example, switching from MSC to MTP mode or vice versa.

For older players - e200v1, c200v1, c100, m200 - the files become hidden files if the modes are switched. An easy workaround is to edit the folder options to show hidden files.

STEP 1 - Edit the folder options of the computer to show the hidden files

Windows XP:
1. Double-click My Computer.
2. Under the Tools menu, select Folder options, then click the View tab.
3. Under Hidden files and folders option, select Show hidden files and folders.
4. Uncheck the Hide protected operating system files (Recommended) check box.
5. Click Apply, then click OK.

Windows Vista:
1. Double-click Computer.
2. Click the Organize button on the upper left corner.
3. Click Folder and search options.
4. Click the View tab and select Show hidden files and folders.
5. Uncheck the Hide protected operating system files check box.
6. Click Apply, then click OK.


SOLUTION 2 - Perform a Check Disk.

STEP 1 - Switch to MSC mode
1. On your Sansa player, switch the USB mode to MSC.
2. Connect the Sansa player to the PC.

STEP 2 - Perform Check Disk
1. Locate the Sansa player under My Computer.
2. Right-click the removable drive associated with the Sansa player, and select Properties.
3. Click the Tools tab, and then click Check now.
4. Click to select the Automatically fix the file system errors and Scan for and attempt recovery of bad sectors check boxes.
5. Click Start.

See also this tip on what it all means http://www.fixya.com/support/r3774047-solve_mp3_player_connection_problem