Before I describe the problem, I'd like to specify that I'm not confused about the size of the partitions (they haven't been extended yet, so of course they haven't grown) but the size of the actual device GParted sees.
I had a
vdi file with ArchLinux installed. The size was 8GB. I extended this to 20GB, which I can confirm in the VirtualBox GUI on my host.
However, after having loaded the GParted live cd into the VM and booting into it, it says that
/dev/sda is an 8GB device. It won't let me resize anything because it doesn't think the drive actually got any bigger.
Is there something else I should be doing?