If a backup set is expired, what can you do to correct the problem?
A. Change the retention criteria.
B. Make the lost backup set pieces available to RMAN again.
C. Run the crosscheck command to correct the location for the backup set piece contained in the
metadata.
D. Nothing. The backup set piece is lost forever.
E. Call Oracle support, their assistance is required.
You would make the lost backup set availbale again by running the crosscheck command once the backup set piece becomes available on the backup media.
原文：http://www.cnblogs.com/Babylon/p/7815788.html