? OS (Not available)

When I look in Ghost > Advanced > Drives, one of the entries is a blue ? icon followed by OS (Not available). Its status is shown as unavailable. Is it safe to remove this item?

 

The other entries on the screen are a normal entry for OS (C:\) with a status of backed up, and an normal entry for my external backup destination.

 

Does anyone know where this blue ? item came from?