I am aware of hard drives failing, not partition failing. If you want to have back up, have it on a separate drive. As far as i know, when a hard drive goes down, all partitions go with it.
the "sda", "sdb" thing shouldnt matter, you would have seen a problem if it did.
partitions do fail too, but is more common to see a ntfs partition fail than a ext4
about having a backup on the same hard disk, it is not secure but hard disks normally last for 5 years with decent performance unless you got a special model(ready to die in 6 months
about swap, 3.8 gbs is big, almost too big, how much ram do you have there? the rule is to have 1.5 times the size of ram in swap but if it is a desktop and you have more than 2 gbs you will not use that swap, possibly never
just leave 2 gbs of swap
about having multiple parts of the os on different partitions/hard disks i never done it but in the past this was a common practice, specially with swap with pcs with low amounts of ram
i personally don't have more than two partitions, one swap and one /
/ contains /home as a folder, all fine here since 2007 with pclinux( all was fine since 2005 with mandriva)
have multiple partitions sometimes just means that you have multiple partitions, nothing more, no benefits, no improvements, nothing more
if that hard disk was mine i would make a swap at a right size, a / with 400 gbs and the rest for a ntfs partition to share with my second os, xp