How it works
RAID combines drives in an arrangement managed by hardware or software. Different levels have different goals and failure behaviour. RAID 0 distributes data without redundancy; RAID 1 maintains mirrored copies. Parity-based arrangements use additional information to tolerate certain drive failures, within their design limits. The usable capacity and performance depend on the chosen arrangement, drives and controller rather than simply adding every drive's label capacity.
Redundancy is not a backup. Accidental deletion, malicious changes or corrupted files can affect the active array and its mirrored data. A failed drive can also leave an array in a degraded state, where another fault or an unsuccessful rebuild threatens the remaining data. Replacing a drive does not instantly restore protection. Controller settings, compatible replacements and the manufacturer's recovery procedure matter throughout rebuilding.
Practical checks
Identify the actual RAID level, member drives, controller and backup before maintenance. Keep a separate, recoverable copy of important data and test its restoration. Follow alerts and documented replacement procedures; do not remove a drive based solely on a guessed bay number. Avoid creating or initialising a new array over existing data when investigating a fault. After recovery, verify array health, backup operation and the affected files. If the arrangement is uncertain, preserve its configuration and obtain a diagnosis before changing membership.