| First up, RAID = Redundant Array of Inexpensive Disks. It was inexpensive compared to the alternatives at the time RAID was a new concept. There's nothing random about a RAID array, which really needs all disks to be identical in order to acheive the best performance.
It's definately not 'just a way to make use of old drives'. It's primary use is in the first word - redundant. People who use a RAID array seriously require that redundancy. They can't live with the downtime a dead disk would represent. For some companies, an hour of downtime represents a loss of more than $100,000. If it took three hours to do a restore, that's a huge loss! Mind you, people who use RAID often use other redundant kit too. Having done work in this area in the past, it's not usual to see redundant routers, databases, webservers,firewalls, application servers, switches, load balancers, power feeds, power supplies and internet feeds.
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