1. License requirement (click on link)

2. Which drive has the most space? OSX system drive or another drive? If the other has, try making space on OSX drive by moving some files across. I moved many of my programs across & keep my iTunes library on an external drive.

Other than that, do a safe boot, repair permissions on your OSX drive, and Repair disk on the other drive. There may be a directory corruption. Most common corruption is file info such as how much space the files are using up on that drive. SAFE Boot & Repair Permissions

* Sorry for the late reply.