Since i don`t see any replies here, would post what worked for me.
In or situation clean installation was not an option due to multitude of custom configurations that would be practically impossible to recreate.
OS – Server 2008 R2
2 hard drives, mirrored, 1x system reserved and 1x data partitions
Data partition got corrupted on drive 1
Only secondary plex was loading when both drives are on
Disconnecting disk 1 and trying to boot from disk 2 did not work
Removing mirror on data partition was not an issue,
System Reserved partition did not work with same error as OP (part of the boot or system)
We ended up connecting third drive and cloning system using AOMEI Backupper cloning functionality that took a working system reserved partition from drive 1 and data partition from drive 2
I am sure there are other applications that can work just as good on that method
After that i have connected only cloned drive and reformatted MBR.
Worked, no mirror left to speak of, OS loading from single drive, all configurations and databases online