Problems remounting an external drive after unmounting it

My mp3 player (iRiver) will not mount after having been unmounted. I literally have to reboot the system to get it to detect it. It shows up under Places > Computer, but when I double click on it, I get these errors:

Unable to mount the selected volume. Error: device /dev/sda1 is already mounted to /media/H300 Error: could not execute pmount

Any ideas?

Error: device /dev/sda1 is already mounted to /media/H300 Sounds as if it's already mounted. Did you look in /media to see if there's a folder called *H300* there?

Are you sure it unmounted?