An SSD isn't a good choice for backup for lots of reasons.

If you are concerned about data loss then I'd just purchase another external hard drive (or invest in a NAS solution of some sort - given HD prices have gone up so much, you might only need to spend an extra few pounds to get a NAS with the storage in it).. and use that

i.e., something like this
http://www.google.co.uk/search?q=1tb+nas&hl=en&tbm=s...