If you have successfully unmounted a partition the kernel has to tell you that the device isn't mounted if you issue the same command again!
A partition, as in your case, can be unmounted by "umount" command either with
The kernel does not need to know the other parameters for unmounting it. It will still be shown in "fdisk -l" until you unplug the device or power down it.
In Linux you can manually mount and unmount a device (partition) any number of times. Issuing a command "ls /media/usbdisk" should tell you the status of it or if you open a file manager in the desktop you can actually see the files disappear and re-appear in front of your eyes.