People planning their storage needs might want to read
this white paper (PDF format) from Wiebetech. It discusses the various ways that disk mechanisms can be configured in an enclosure (irrespective of how they're accessed) and gives a statistical treatment of the various configurations' risks of unrecoverable data loss purely as a result of hardware failure.
One key finding is that some multi-disk configurations are more reliable than a single disk, while others are drastically less reliable. And since hard drive manufacturers often don't advertise what drive-mechanism configuration is inside the enclosure, you may not find out you've got the drastically-less-reliable variant until you suffer complete, unrecoverable data loss. (Ask me how I know this...)
Naturally the examples in the white paper mostly reference Wiebetech drive enclosures, but since the statistics deal solely with mechanical failure, they should apply to any type of enclosure. A few factoids are out of date since the paper was written in 2006 (for example, at that time 500GB was the size limit for drive mechanisms, while 1TB mechanisms are widely available now) but the basic principles are still worth understanding.