This probably comes as a complete surprise to everyone [rolleyes] but I can't install Mavericks.

I get this message with the install button grayed out. [Internal HD I operate from]

This is just an extra disk with movies, etc.

This is Time Machine backup disk.

These excluded time machine backup.

I repaired permissions. Rebooted iMac. Tried again.Turned off Time Machine and tried again -- no luck. Time Machine is working perfectly. But it thinks I'm using two disks.

Any suggestions to try?