But useless is a disk fails..
Nope it only takes two dead drives to kill a RAID array so long as one of the dead drives is the parity drive
So Im guessing you have never seen any of the modern raids? with multiple hot spares or raid ADG have you? Raid 10? You're not looking like you really understand raid. Heck with a raid ten you could lose half your disc and not lose information.. and thats a baisc raid 10..
Not necessarily. If people work locally then copy to a central repository if the repository craps out you should have the latest out there somewhere, much better chance of data recovery then than if everything lives exclusive in the repository.
I've seen Raid 10, it all lives and dies by the parity disk, if the parity disk lives you can reconstruct your data with otherwise massive disk loss, but the parity disk is the key to recovering the data, no parity equals major uh oh.