Step 1.) Upgrade to Debian 11 first
The process to go to Debian 12 is not as smooth as 11, when trying to upgrade from Debian 10. In fact, it doesn't work directly, so you'll first need to follow this guide to update to Debian 11, reboot and come back here if successful.
Step 2.) Update sources.list
Update your /etc/apt/sources.list like this:
deb http://........
mdadm --create /dev/md0 --level 1 --raid-devices 2 /dev/sdb1 missing --metadata=0.90
mdadm: super0.90 cannot open /dev/sdb1: Device or resource busy
mdadm: /dev/sdb1 is not suitable for this array.
mdadm: create aborted
Sometimes running "partprobe" can fix this. Other times it requires a reboot.
One other manual thing that can be done is the following to fix it (if dm is using and blocking it):........
This was caused by some weird dmraid setup which kind of takes control of drives even if they're blank/unused.
1. Check the table.
dmsetup table
ddf1_44656c6c202020201000006010281f0b3f5195b77cf86172: 0 3905945600 linear 8:0 0
ddf1_44656c6c202020201000006010281f0b3f5195b77cf86172p3: 0 37124096 linear 253:0 284547072
ddf1_44656c6c202020201000006010281f0b3f5195b77cf86172p2: 0 283496448 linear 253:0 1050624
ddf1_44656c6c2020202010........