Image Tool Can't Read Drive (v.4560)

Installed the latest release, and subsequently installed parallels tools.

I run Image Tool and try to resize the drive from 12000 to 18000 mb's. It runs for a period of time and the bar moves, but when it gets 3/4 of the way across the screen I get an error message that the drive cannot be read and I need to select another (paraphrasing). Any ideas? Thanks.