I use openmediavault for my home NAS, it does all the heavy lifting for you with a nice GUI so you don’t have to config everything yourself. I’d recommend checking it if you don’t have a lot of experience with RAID setups or don’t want to do manually.
Just please remember that RAID is not a backup solution, it’s a redundancy solution. If you have data corruption on one side, it can copy itself to the other mirror and then you’re screwed. If your media is important, keep a proper separate backup.
Maybe it still appears in
/var/lib/dpkg/status
? If it is, try deleting the entry from there.