How to resize VM MUCH faster, without a giant temporary file, and on any disk.

At first I was having a problem resizing a virtual expanding disk image. It was on an external drive, so imagetool wouldn't work. I then copied it to my main drive, which took quite a while, and attempted imagetool again. It still would not work properly, because it needed to create a huge temporary file and my main disk didn't have enough space left after copying over the VM. Even if I did have enough space, it was looking like it would take about 30+ minutes (the VM was about 20GB).

I was able to figure out a solution that took about 20 seconds (after figuring it out). In case it is helpful to anyone, especially the Parallels developers, I have it below:

First and foremost, I cannot guarantee this will work and won't cause any problems. I would not recommend you do this with important data that is not backed up. What I can say is that it worked fine for me, and has not caused any problems.

First, you need a hex editor. I used hexfiend. Locate the image you want to resize. In Parallels create a new virtual hard drive of the size you want the first one to be. Open both of these images in the hex editor. The top three lines are what matters. Copy the data indicated in this picture from the new image with the right size to the image you want to resize. It is only three sections.

Now, check the image in Parallels. It should show up as whatever size you changed it to and work correctly.

If any Parallels developers are reading this, perhaps you could use this method to resize disks in a future version of Parallels. The current way isn't very good.