What does this mean: Appendable media

Appendable medias are media which can be used to add data to it. Once some data written to a media, it will become appendable. New, Blank and erased medias are not appendable but overwritable. 

When a job starts, BE will search in the target media set for suitable media according to the way job is configured. According to  "No appendable media could be mounted.  Switching to overwrite operation on scratch media." this message, your jobs are configured to start as Append to media, overwrite if no appendable media are found. So, if no appendable media are found in target media set then BE will use recyclable media to continue the job.

According to media management setting configured, it will choose between recyclable media available in target media set and SCRATCH media set.

An explanation of Overwrite Protection and Append Period: http://www.symantec.com/docs/TECH5459

When medias become appendable: https://www-secure.symantec.com/connect/forums/sbe12-keep-data-4-weeks-append-period