I feel cozy:
Ubuntu documentation:
"hdparm --dco-restore
Reset all drive settings, features, and accessible capacities back to factory defaults and full capabilities. This command will fail if DCO is frozen/locked, or if a -Np maximum size restriction has also been set. This is EXTREMELY DANGEROUS and will very likely cause massive loss of data. DO NOT USE THIS COMMAND."
While project was permafrost state, RAID box was a donor for son's playmachine, and one disc is now renewed by identical "new old stock" unit. Pedantic soul would check, that setup is similar for both disks. "Fdisc" does find both disks, and it is bit questionable to fix something is already working....