The dynamic disk allocation of virtualbox means that if you set 10G, it will not occupy 10G at the beginning, and it will gradually become 10G as the usage increases. But when it reaches 10G, it can no longer grow. At this time, the hard disk needs to be expanded.
cd “C:\Program Files\Oracle\VirtualBox”
VBoxManage modifyhd "D:\VirtualBox VMs\ubuntu16-64-10\ubuntu16-64-10.vdi" --resize 20480 
The above picture means success.
Next, create, format, and mount the hard disk in the ubuntu operating system, refer to the link below.
refer to
http://blog.csdn.net/ouyang_peng/article/details/53261599
http://www.linuxidc.com/Linux/2015-03/114977.htm