I had the same problem. Aspyr Support actually responded to me and said you need to remove all mounted and virtual drives (I had to unplug my external hard drive and it worked). They also mentioned you need to make your virus scans like Norton, etc aren't blocking. Here's there response:
"If you are receiving an error message upon launch that says something to the effect of
"please insert disk" (even if the disk is inserted), you are very likely
experiencing a conflict with the copy protection software on the disk itself. Any
software that places virtual drives on your system (for the purpose of disk imaging, CD
burning, download managers, even some antiviruses and firewalls) WILL cause this issue. Example software would be DaemonTools, Alcohol 120, GameJack, CloneCD, GetRight, Agnitum Outpost Firewall, Panda Antivirus, or Norton Antivirus. Please uninstall these titles, or at the very least disable them in your Task Manager."