I am using oracle VirtualBox, my host is ubuntu 16 LTS and my machine is windows 7 ultimate 64bit. I'm trying to add some storage to my machine, but I couldn't find a way to expand the vdi file I currently use, so i tried to add another one, but the machine doesn't seems to recognize it.
Any suggestions?
It's not clear what you did to add a vdi file (a "disk") to your vm, nor what you did that you didn't see it where you expected. Adding a new vdi is the virtual equivilent of plugging in a new, unpartitioned, unformatted hard drive. In your Win7 vm, open control panel > admin tools > computer management > disk management. You should see a new disk waiting to be partitioned and formatted.