Raid 5 is the choice if you just can’t afford to do a serious mirrored array (I’ve worked at places like this). It’s also fine if you can tolerate down time and really want extra storage from a fixed number of disks.Evidently, availability of a storage system is not that great if it consists of disks in RAID-5 configuration. I am not operating RAIDs myself, but from what I read already some years ago, mirrored schemes are the way to go nowadays if availability is desired (and disks required).
/armchair storage expert mode off
It’s also possible some current sysadmin is paying the price for the mistakes of a past admin. All seem possible.