we ARE talking about redundancy when we talk about backups. we backup so if the main hdd fail, we can still use the backups. isnt that redundancy? also RAID is pure mirroring, no indexing like RAID 5 and above, so if RAID controller or one of the hdd crashed, you still can plug into a pc to use. you just end up with two identical disks.
using 1 hdd to backup 1 hdd is the same as RAID 1, just need twice the time and double the electricity.
btw, although the maximum theoretical size of a FAT32 partition is 8 TB, Microsoft has voluntarily limited it to 32 GB to promote NTFS. unless you use other OS.