Step 1 is to increase the size of the file itself. That can be done with the VBoxManage utility found in the c:\Program Files\Oracle\VirtualBox directory. Run the following:
VBoxManage modifyhd YOUR_HARD_DISK.vdi --resize SIZE_IN_MBStep 2 is change the partition to take up the extra space created in Step 1. That can be done with the DiskPart tool that is built into Windows. To do this, Launch the virtual machine, go to a command prompt (Administrator Mode), and run:
diskpartGet a list of the volumes mounted on the machine:
list volumeFrom the resulting list, get the number of the volume for the one you want to extend. Select the volume:
select volume xWhere x is the # you got from the list volume command. Verify you have selected the right volume:
detail volumeExtend the partition to take all available space:
extendShut down the virtual machine, disconnect the volume, and it should be ready to mount to the original virtual pc.