Raid

I’m trying to find out about RAID- in 1. can you put extra files/folders on
just one disk- can one external HDD be turned off (I have two WD 750GB SATA HDDs with eSATA)- do they both have to be on to read the file… recorded & if so how do you know if one disk has failed, on GA-EP45-UD3R can you setup disks you want to use, the raid level (0-10) ? or can you tell me links where I could find out, I’ve been fruitlessly Googling for hours but I obviously didn’t know the right place to look.
Thanks

Worth a view: http://en.wikipedia.org/wiki/RAID