When creating a dynamic disk VirtualBox asks you to specify the maximum size. If the max dynamic disk size needs changing after its initial creation we cannot use the VirtualBox interface, instead we need the VBoxManage command line tool.
1. Shutdown the VM and quit VirtualBox
2. Open the Terminal app and use the following command to navigate to the VirtualBox app directory
3. Now in the proper directory, you’re ready to run the resize command with the following syntax:
VBoxManage modifyhd –resize [new size in MB] [/path/to/vdi]
VBoxManage modifyhd –resize 30000 ~/Documents/VM/Windows10.vdi
4. If desired, verify the change has taken place with the showhdinfo command:
5. Launch VirtualBox and boot renewly resized VM