G'day
@Peer - you have me wondering whether that
- is just a Live USB stick of Debian or
- do you have it fully installed to hard drive?
Or do you have a full install of Debian on the stick?
Although you do very well with English I am mindful that it is not your first language (Peer is German), so ask any questions if you are not sure of what I am seeking to establish.
I know you had Ubuntu before you started learning about, and working with, Debian. Is/was the Ubuntu on a stick as well?
Is it just Windows on your PC?
I am thinking back to late May/early June at this Thread of yours
here, and in particular, this Post of Stan's -
https://www.linux.org/threads/debian-live-usb.17921/#post-54228
If you have not changed that since, then you have a Debian stick with Persistence, which saves your changes and saves your Settings, but it is not a full install and will not show up on a Grub.
Can you give us a screenshot of GParted, showing the partitions available?
Wizard