To avoid this error:
nb2:~nico/vcs/ungleich-tools# sudo chroot /tmp/tmp.U3RiZH2Nu9 /bin/sh -c 'grub-install --target=i386-pc /dev/sda'
Installing for i386-pc platform.
grub-install: warning: Attempting to install GRUB to a disk with multiple partition labels. This is not supported yet..
grub-install: warning: Embedding is not possible. GRUB can only be installed in this setup by using blocklists. However, blocklists are UNRELIABLE and their use is discouraged..
grub-install: error: will not proceed with blocklists.
See also:
https://askubuntu.com/questions/666527/install-grub-claims-that-i-have-multiple-partiton-labels-and-that-embedding-is