i think its got something to do with bad sectors or corrupted partitions. This can occur for different reasons incl. virus attack, power-surges, software installation, driver issues, loose connection or partitioning software.
Also is there any write-protection for the mp4 player? That might be a cause.
similar probs come up when a bad floppy is accessed... it asks for formatting the floppy, but is unable to do it as the disc is being accessed whenever put in, till it gets understood completely by the computer, which'll never happen due to bad sectors... a deadlock situation.
Another chance is that the files being corrupt initially, thus taking time to read & the resulting "hang-up". Or the USB Mass Device drivers being corrupted. A fresh driver install might help.
You didn't mention the operating system.
Try scandisking completely. If corrupted, data wont be completely retrived. Also the format is FAT32, not FAT... which is an old format.
Formatting should be the last option tried. Also go for Full Format, not quick format. Try the mp4 player's format option. Check manual for details.
Or try the FDISK tool in DOS, or the Active@ KillDisk sw or other formatting utilities is everything else fails.
If problem still continues, come back to this post with full details as to what u've done about it yet! Someone might help u out. Hopefully.