raid 1 0

Hi,

I have 4x4TB hard drives setup as raid 1+0 giving me 8TB mirrored. Ive recently had an error come up saying I need to repair the disk. When I repair the disk it says 'Disk Utility can’t repair this disk. Back up as many of your files as possible, reformat the disk, and restore your backed-up files'. The reason I went 1+0 was to stop this happening, so I could replace the drives as they fail and rebuild. How can the whole setup fail and need to be backed up?

Thanks in advance 🙂