sorry to butt in but when 2external in RAID mode, it will function as mirror to another one, right? its like we're creating another copy of the content in external 1 in external 2, right?
and if one fail, the file in another ex is not affected...is that right?
so, how to use RAID? is it difficult to set up or difficult to use something like i have to format 64GB memcard to FAT32 to use in Galaxy S2 and have FAT32 restriction
Generally, you can't (or rather, you shouldn't) put external drives into RAID. Drives in a RAID array must always be connected. It's not at all the same as just copying one entire disk to another, because you must have your RAID controller to read the disks properly. The biggest difference between RAID 1 and just straight-up cloning a disk is that with the second option you can bring one of your disks anywhere, plug it in anywhere, and it will be fine as long as the computer you use supports the filesystem on the disk. You can't do that with RAID 1 unless the target computer has a RAID controller too, and even then it's not really recommended to do that, because that's not what RAID is for.
Formatting to FAT32 has nothing to do with RAID. RAID is a low-level technology that concerns how your data is spread over multiple disks, and provides an interface to read the collection of drives like it was a single disk. FAT32 is a type of filesystem you put on whatever storage device you choose to (though it is not recommended to use FAT32 on a RAID array because of its limitations) and defines how your data is stored in the device, which can be a RAID array, a single hard drive, a USB drive, whatever.
Anyway, @OP, I think what I would want to tell you has already been posted. Blu-ray media is not ideal for data backups - use a second hard drive instead. A hard drive does not deteriorate nearly as quickly as Blu-ray media.