virtualbox.org

I've followed the FAQ for enlarging a virtual disk, but I'm still having trouble. The setup: MacOS High Sierra host, Win 7 guest. The steps: I successfully run VBoxManage, getting the 0%... 10%... etc, all the way to 100%. Then when I start VirtualBox, the new hard drive size displays correctly under the storage details. However, when I fire up gparted, it still only shows the original disk size as available to allocate. The Win 7 guest also has a partition management tool installed, and it likewise only sees the original value of disk size available to the partitions.

I'm a bit baffled. Any thoughts?

and now you get to do Step 2

It's all in the faq: Howto: Resize a Virtual Drive. Note in particular what it says about snapshots.