4 disks in RAID5: this protects against a single disk failure, ie whenever 1 of the disks fails. RAID configurations determine how the data is distributed across the 4 disks and how many disks may fail before losing data while taking care of specific requirements for fast read/write. The more protection, the more overhead data absorbed for RAID. RAID5 I use. RAID6 protects against 2 disk failures. Keep in mind: When purchasing all hard disk in the same RAID set, at once from the same supplier, it may very well be that these disks are produced in the same batch at the factory. Any production error will likely be applied to all disks in that production run. Also, the lifetime of disks from the same production run is more or less equal. When 1 disk fail, it's likely the 2nd disk will fail soon as well. Cases are known, two or more disks from the same production run, fail almost at the same time. That's why professional installations, mix disks from different production runs. In RAID sets, HDDs (or SSDs or NVME's whatever disk type is used in the RAID set) are to be of the same size and specs to make sure the RAID set performs best. Sure, you don't have to go for a NAS at all, but use 1 or 2 external HDDs to store and backup your files, but a NAS saves you a lot of headaches. That said, the RAID configuration is to be applied and carefully chosen. I can help you with this if you like. IN the end it all boils down to effort, and your own requirements what the best solution is for you :)
