there are some people that sugest that it is actually better to have a hard disk in use permenantly as if it is rarely used it may be more likely to seize up and fail.
I don't really know that much about hard drives but I would probably go for sata and if you can afford it and are really concerned about backup then got for a RAID NAS device as this provides the best guarantee of security
this is the best at raid 5 but is a bit pricey lol -
http://cgi.ebay.co.uk/Buffalo-TeraSt...QQcmdZViewItem]
raid 1 can also be used for redundancy and is much cheaper -
http://cgi.ebay.co.uk/NEW-DUAL-NAS-R...QQcmdZViewItem