I have a cFast USB 3.0 (eSATAp) flash disk and I am trying to install Windows 7 to it. When the installer displays partition list, my cFast drive does not appear. It is formatted as a normal NTFS disk and shows up properly under computer local disks.
While installing Windows 7 you get offered to load third-party Drivers. In the bottom-left corner it says "Load Driver". You should check your USB Device Manufacturers Website for additional drivers.
After spending some time with partitioning finally found a solution. When partitioning the flash disk (cFast) using built-in windows disk management, in order to be able to install an operating system to it the partition must be marked as Primary and Active.