Mavericks problem! "Disk cannot be used to start up your computer"

Hi there,

I finally managed to download the Mavericks Installer from the App Store. When I want to install it I can’t select my Hard Drive because it’s greyed out and says "This disk cannot be used to start up your computer". (Not the Time Machine Message!)

I’m on a MacBook Pro 15" late 2009 running Snow Leopard (10.6.8). I do not use File Vault, I had a Windows Bootcamp Partition until a few weeks back but deleted it.

What I did: I went to Reboot Mode and run Disk Utility (Verification + Permission Verification) everything seems fine. I also tried the resizing (partitioning) of my HD with Disk Utility, it won’t let me do it. I’ve tried several different amounts of space.

I also have about 45GB free space, so that shouldn’t be a problem.

I have a Time Machine Backup but really want to avoid formatting my disk.

Would be very glad if anyone could help me please!