Recently I went to start up my VirtualBox Windows 7 client and I got the following error:
"Hard disk '____.vdi' with UUID {___} cannot be directly attached to the virtual machine 'Windows7′ ('___.xml') because it has 1 differencing child hard disks."
I have no idea what 'caused this error and it took me a lot of searching to find a good solution to it. Since it took me so long to figure out I decided I would post the solution here for my own reference and hopefully to help anyone else that faces the same issue.