I don't know where to start with this. The Drobo does not even include hard drives and it costs $400. You still have to install the hard drive(s) of your choosing so why mention "Not if the external drive fails"? You'd be choosing brands/makes/models of hard drives just like you would if you bought an external hard drive without a Drobo. Let me say this again for anyone not familiar with Time Machine. This is how easy it is. The very first time you plug in your hard drive to a usb or firewire jack a dialog box will come out asking if you would like to use this drive as a backup drive. You click yes AND THAT IS ALL YOU EVER HAVE TO DO. There is no messing around with software or anything. You can go into preferences and choose the time of backup and what folders/directories you'd like to backup if you don't want to backup the entire machine but that's about it. And it's FREE. It comes with your Mac. The Drobo may look nice but it seems completely unnecessary.
http://www.apple.com/macosx/features/timemachine.html
http://www.apple.com/macosx/features/timemachine.html