Installing 20.10 on a multi partion system

[Larsson]

Have 4 disks with 10(?) partitions. Installed on sdc1 as a full disk. Start up to be placed on sda.
When finishing, the Grub reports it can't install start up at sda.

ProblemType: Bug
DistroRelease: Ubuntu 20.10
Package: ubiquity 20.10.13
ProcVersionSignature: Ubuntu 5.8.0-25.26-generic 5.8.14
Uname: Linux 5.8.0-25-generic x86_64
NonfreeKernelModules: zfs zunicode zavl icp zcommon znvpair
ApportVersion: 2.20.11-0ubuntu50
Architecture: amd64
CasperMD5CheckMismatches: ./pool/restricted/n/nvidia-graphics-drivers-450/libnvidia-fbc1-450_450.80.02-0ubuntu1_amd64.deb
CasperMD5CheckResult: skip
CasperVersion: 1.455
CurrentDesktop: ubuntu:GNOME
Date: Tue Oct 27 18:20:50 2020
InstallCmdLine: BOOT_IMAGE=/casper/vmlinuz file=/cdrom/preseed/ubuntu.seed maybe-ubiquity quiet splash ---
LiveMediaBuild: Ubuntu 20.10 "Groovy Gorilla" - Release amd64 (20201022)
ProcEnviron:
 LANGUAGE=nb_NO.UTF-8
 PATH=(custom, no user)
 XDG_RUNTIME_DIR=<set>
 LANG=nb_NO.UTF-8
 LC_NUMERIC=C.UTF-8
RebootRequiredPkgs:
 linux-image-5.8.0-25-generic
 linux-base
SourcePackage: ubiquity
UpgradeStatus: No upgrade log present (probably fresh install)