Time Machine runs out of memory?

I wanted to create a backup of my laptop via Time Machine again. So I connected my external disk as always. However, it shows me that only 12mb are free. It is a 1 TB hard drive with previous backups deleted, so it should be completely empty. I also can't find any other files that I could delete. How do I get the plate completely empty again?

Go to the hard drive under "this PC" and see how much memory is actually free