raid five is pretty much useless with only two drives...
it works something like this...
raid 0 spans two disks without parity...
so disk A is 100 GB disk B is 100 GB you get a total size of 200GB and the data is stretched across the two disks...
obviously the more disks you add to the set the larger it gets, (though you have to add the disks as you set up the array.
raid five is a little more complicated, it strippes the disk the same as raid 0 does, but it also adds parity
so if you have 100gb disk A 100gb disk b 100 gb disk c 100 gb disk d and 100 gb disk e
disk a has 4/5 space devoted to data and 1/5 partity checking, the same across the other disks...
so whereas you have 5 100 gb disks, you'll only ever be able to use 400gb since the size of 1 disk will be taken up in parity checking...
there are several plusses to this, and a few negatives.
negatives are, you loose diskspace,
and it takes longer to write since parity has to be calculated.
positives are, it's faster to read from a raid 5 disk set,
and 1 disk can fail entierly and you will not loose data, data can be (mostly) contructed from the the other parity partitions.