Easily increase your VirtualBox VDI hard disk size

1.Create a new Virtual Machine in VirtualBox. Make sure that you set hit General -> Advanced and duplicate your original machine’s settings here. For me, the key was setting the same IDE controller type, such as PIIX3, and setting the same OS. Just used this to increase the size of my Windows 7 disk from 20 to 100 GB.

For example, my original Machine is Win7, system size is 20G, my new Machine is Win7-2, system size is 100G

2. Clone the original machine to new machine, using command :

$: VBoxManage clonehd /...path.../Win7.vdi /...path.../Win7-2.vdi --existing

3. Now Login your new machine Win7-2, you will find the disk size c: is still 20G, but not 100G.

You should use disk management tool to increase the size of C: partition. All Control Panel Items --> Administrative Tools --> Computer Management --> Storage --> Disk Management --> Choose C: partition --> Click Right key of mouse, and choose "Extend Volume", now you can add unallocated 80G to your C: Partition.

Good luck.