Check the last reply on my thread Unable to boot after installation (Dropping to a shell!) I think you may have the same problem which I did.Definitely need to defer to you on these things, as its way above my expertise. In any case, I now think my issue is slightly different from the others here. I'm failing to boot due to the mdadm message others are getting, followed by busybox prompt, But in my case, I was starting with a fresh install and no disks connected. I wanted to pull some baseline power numbers with no disks attached, and couldn't get it to boot due to that message. I never got to the point where I connected the disks, so different situation than failure to re-assemble an array.
What is really strange to me is that I haven't used RAID in years. Not since @ryecoaaron recommended I try rsync instead of RAID quite some time back. Now I'm using Snapraid, so there's never been an array to assemble or reassemble, or have any other kind of issue with!
Working with computers since the days when unboxing and set-up required 3 weeks with a soldering iron!