@nobody It's not completely redownloaded. It's like a tower that you keep adding blocks to to make it taller. If you already have every block from a while back, you just need to be updated. The blocks aren't that large.
@nobody RAM volume does not really matter.
If you loose 1 drive of 4 - miner will try to get access to it, but other still works fine and will mining at 3 drives. http://imgur.com/a/0llBI my M:\ drive is dead yesterday
If you have many plots at 1 drive - you will loose 8ms per each for get access to it