@Pete
From Tools\Options\Media Management verify that the option to "Use scratch media before Overwriteable Media in the Target Media Set" is selected rather than "Use Overwriteable media before scratch. This way, even i fhte BKF files are past their OPP, new ones will be created until the disk fill up or the Disk Space Reserver is met
@JF
None of the properties of the B2D device affect whether BKF files are overwritten or not. max number of sets per B2D file merely tells BackupExec whether to start a new BKF file every time a new backupset in a job is encountered. a new backup set is created for each resource in a job - Share, data base etc.
Allocate max size for B2D files just allocates all 4GB (by default) when a new BKF file is created, rather than a smaller amount, to be incremented if the job needs the whole 4GB. This helps reduce fragmentation of the disk the device is on, but can slow down the job if multiple BKF files need to be allocated, especiall if you have raised the maz size for BKF files