BackUp Full

I have a 4 TB back up disk (that is actually mirrored by another 4 TB drive for added security) and a 3TB hard drive in my iMac.

Time Machine is now claiming the backup is full and needs 5.85 TB to complete the latest backup?

As far as I can see I have excluded the extra storage drive from backup. So how can the latest backup require 5.85 TB when my hard drive is only 3TB?

Why are old backups not being deleted to make room?

Should I delete the backup folder completely and start a backup from scratch, making sure that the extra storage drive is excluded from backup?