The error message indicates that it can't find the correct media for the second tape/bkf file which could mean
- you need to inventory the second tape
- you have an incorrect second piece of media
- or it is correct but another job overwrote the media at a later time - which would make it part of a different family and backup set.
- there is a 3rd media relating to the family that you need as well as the 2 you already have (for instance it could be that your first media is actually sequence number 3 in the family and it needs the media that came before to run the catalog job - this would potentially be a scenario if you run append jobs and never do overwite jobs.
To break down what you posted:
"Backup set #3 on storage media #1" indicates that it was in 3rd set on the first media
- a set in this instance means a drive letter, a system state etc so assuming that you say had 2 drives and backup C: D: and System State in that order that Set #3 would be the System State.
- if you do append jobs then the set could be the first drive letter in the second job...
"Media for Sequence number 2 in this media family has not been inventoried" means it is looking for a media that is not present in the inventory of the server.
As I have a feeling the Media #1 would equal Sequence Number 1 - it would appear to be your second tape that has either needs to have an inventory job run, haw been overwriten or is the wrong tape.