RD1000 cartridges - re using

I am using Dell RD1000 tape cartridges, 1 for each day of the week. There are 3 servers being backed up each day and they take up about 80% of a cartridge i.e. about 800GB. The backup job is set to keep the data for 1 day but when the following weeks backup comes round backup exec does not free up space on the cartridge but just fills up the remaining space before asking for a new cartridge. I have tried erasing the media beforehand but it doesn't delete everything. Currently I have to format the cartridge in Window and then I stand a chance of it working.

Am I missing something? The storage is setup as a disk storage device which mentions the DELL RDX range so I would hope it is correct.