As mentioned in a post in a different thread, it could be anything or nothing. Maybe the OS and related server updates were on the same disk. Eventually it will be made public but not all of it has to be related to the running of the actual mixing part of the website. Could it be 7TB was the size of the disk but the used space was much less?
As for the 7 TB: we know absolutely nothing of what ChipMixer held there (IIRC), besides private keys (which if I remember correctly were not supposed to be deleted). The only thing I remember highlighted is that they delete logs.