RAID array

RAID puts several drives behind one volume so the machine sees a single disk. Mirroring writes the same data to two drives; parity schemes spread data across several and store enough extra information to rebuild whichever one fails. What it buys is uptime and survival of a single failure, and an edit bay that keeps playing back while a replacement rebuilds. Drive sizes have changed the maths, because rebuilding a 20TB disk takes long enough that a second failure during the rebuild stops being a remote possibility, so dual parity is now the sensible default. RAID is also not a backup, since it happily mirrors a deletion to every disk at once.