How to merge two directories with failover?

Lets say I have two devices: - /dev/sda1 mounted to / (system partition) - /dev/sdb1 mounted to /media/data (data partition, usb device may be unplugged) I want to merge/overlay/raid two directories like so: - /media/data is the primary directory - /usr/data is the backup/failover directory that exists on the system partition The resulting directory (e.g /mnt/merged) will consist of the above two directories so that: - when writing a file to /mnt/merged the file will be written to /media/data - if the /dev/sdb1 is not available while writing (the usb storage is removed) then write to the backup /usr/data and when the primary partition is plugged again move the data to the primary partition - (optional) setup the second partition as a cache partition in case it is faster than the primary partition, so that reads and writes happen to the backup (faster) directory before moving to the primary directory