Add backup cache settings for when external drive is missing

The Disk Cache settings recommend that a separate drive is selected as the location to store cache files. It might be helpful to be able to set a primary location and maximum cache size for an external/secondary drive, and a backup location/size for when the primary drive is not available.

This way, if you forget your external drive when working out of home, for instance, the cache is still handled in a predictable manner.