Which RAID level fault tolerance allows two disk failures?

RAID 1 mirrors data across two or more drives and tolerates only one drive failure per mirrored pair before data loss occurs in a standard two-disk configuration.

RAID 4 uses a single dedicated parity disk, which provides fault tolerance for only one drive failure; a second simultaneous failure results in unrecoverable data loss.

RAID 5 distributes a single set of parity data across all drives, which tolerates only one drive failure; a second simultaneous failure before the array finishes rebuilding results in unrecoverable data loss.

RAID 6 writes two independent parity values - commonly called P and Q, each computed with a different algorithm - distributed across all member drives, so the array retains enough redundancy to reconstruct all data even when any two drives fail simultaneously. This dual-parity scheme requires a minimum of four drives and dedicates the equivalent of two drives worth of capacity to fault tolerance.