Okay, so I must be stupid.

Here's what I WANT to accomplish:

6 nights a week, write an incremental backup to disk
1 night/week run a full backup to disk

Duplicate each Backup to Disk to tape.

I want it to only keep 7 "days" worth of backp to disk.

BUT, I want it to keep writing to the tape until it's full, (ie,
appendable and then overwrite).

I made 4 diff. media sets.

1 for "nightly backup 2 disk"
1 for "full backup to disk"
1 for nightly dupes
1 for full duplicates

I have a backup policy called "nightly incremental" that is setup to run
every week (but only Monday-Sat is checked).

I have another backup policy for "full" that's set to run every week
(run every 1 week) with Sunday set to Full.

What happens is that the backup to disk just keeps filling up and never
overwrites the "old" tapes. The "all media" shows hundreds of
backup2disk "tapes" from a LONG time ago.

So I'm obviously doing something wrong.