Resizing virtualbox vdi doesn't work

I tried resizing my VirtualBox disk (.vdi) with VBoxManage, but it didn't resize the hard drive. The command while in the root folder of VirtualBox was:

VBoxManage modifyhd "C:\Users\<name>\VirtualBox Vms\Linux Mint 14.1\Linux Mint 14.vdi" --resize 30000.

The output showed 0%...20%....40%...100% looking like it was successful. Now my .vdi in VirtualBox shows the virtual size as being 29.30gb, but the actual size is still 7.90gb. When I boot into Linux Mint, it still notifies me that the hard drive is out of space.

Your hard drive is bigger, but you still have your old small partition on it. Use some partitioning software (for example GParted) to resize the partition.