FYI
i had a similar problem upgrading from 8.10 to 9.04
boot failure occurs after loading mdadm
so i logged into the maintenance console and used the command supplied by the OP.
mdadm --examine --scan --config=mdadm.conf >> /etc/mdadm/mdadm.conf
this command added the necessary lines to mdadm.conf and everything ran fine after that
FYI
i had a similar problem upgrading from 8.10 to 9.04
boot failure occurs after loading mdadm
so i logged into the maintenance console and used the command supplied by the OP.
mdadm --examine --scan --config=mdadm.conf >> /etc/mdadm/ mdadm.conf
this command added the necessary lines to mdadm.conf and everything ran fine after that