HDD life expectancy with respect to the Clariion array

Hello All ,

             Need some quick help please , heres what I need to know ,

- Life expectancy of a HDD (FC/SSD) - any refrences will be appreciated .

- How do I calculate the failure rate of a disk with respect to the array .

I've read the Information Storage Management book and here's what it says .

"Consider a storage array of 100 drives , each with an average expectancy of 750,000 hours . The average life expectancy of this collection in the array , therefore is 750,000/100 = 7500 hours . This means that a disk drive in this array is likely to fail atleast once in 7500 hours .

Source - Information Storage Management by Wiley

In the statement above it says each drive has an  avg expectancy of 750,000 hours ??? really ? which is approx 30k days thats way too much , can someone please explain this to me ...

I just want to calculate the disk failure rate for our clariions !!!