Debian's grub doesn't detect Ubuntu (was: Debian's grub doesn't detect Ubuntu)
This issue is now solved.
I wondered why Debian's GRUB doesn't detect Ubuntu, but Ubuntu's GRUB
detects Debian at #ubuntu-fi-offtopic on freenode and I got answer that
Debian doesn't have os-prober.
I noticed that that package is available on Debian repositories,
installed it and ran "os-prober" and "update-grub" as root and then GRUB