Database replication inside a disk?

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I am new to exchange and I am just learning the ropes.
I read MS preferred article and found information that confuses me.

"This storage class contains Exchange Server database files and Exchange Server transaction log files. These disks are large capacity 7.2 K RPM serially attached SCSI (SAS) disks. While SATA disks are also available, we observe better IO and a lower annualized failure rate using the SAS equivalent.
To ensure the capacity and IO of each disk is used as efficiently as possible, up to four database copies are deployed per-disk. The normal run-time copy layout ensures that there's no more than a single active database copy per disk."


Why would exchange replicate the database inside the same disk? How would that help incase of a disk failure?  

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