This failure describes a hard drive whose data has encountered corruption in very specific locations on the device where critical system files are stored. This corruption usually results in the system not booting, or in the file system not reporting file content as expected.
Data recovery is a manual process and is very labour intensive in these cases. It involves analysis of specific recognisable data structures at byte-level and manipulation/correction of specific binary values to correct the defect. In-lab decryption is also usually a requirement.
These types of data recovery tasks do not involve cleanroom handling, as the drives do not need to be physically opened. This is a particularly specialised type of recovery.