The eject manpages says that eject can unmount devices corresponding to the device (e.g., /dev/sda1) or the mountpoint (which is in your case $HOME/USB/myusb).
If eject $HOME/USB/myusb
is not working. Try eject -v $HOME/USB/myusb
to get more information about what is happening.
Tested eject here. It works as expected. Maybe it is a permission problem?