That looks better now, it should now fill HDD five to zero free space.
Then 6, 7 and 8.
Then overwrite 1 (the oldest recordings, on the first disc).
Then overwrite 2, and so on.
If you keep an eye on them over the next few days hopefully you'll see it follow that pattern.
Obviously your total retention period will now grow as it fills this new-found space.
i.e. what is currently the oldest date you can review will remain the same for a few days, until all the drives are filled.
Effectively your retention period is increasing with each passing day until all the drives are filled.
Once all are filled you'll have a good indication of the typical retention/look-back period you will be able to expect.