You need to make sure that your disks are 100% redundant.
A popular way on Linux to do this is DRBD. Here are a couple of guides on how to do it:
The one stumbling block you may encouter is that you can’t easily replicate EXT3 and EXT4 so you have to look for an alternate file system. The one to use is OCFS2.
See other knowledgebase articles on a good configurations for DRBD and OCFS2.