Theft is also another item to consider with internal/external.
Maybe a tower's a little less easy to get away with than with than a laptop like mine, but I try and add redundancy by locking the external HDD down to the desk (a LaCie d2 with a kensington lock, looped around my metal desk leg).
My hope is that the thief is semi-rational and will take the easy-to-steal-and-higher-value laptop and won't bother with a slightly-harder-to-take locked down HDD which has all my data cloned to it.
Of course, then there's the offsite side of a BU scheme to cover against local disaster such as fire, which I have as yet to fully sort out (annoying to have to cycle offsite HDDs around, and bloody mac iDisk doesn't seem to like to transfer anything more than a few MBs at a time or I'd use it to backup my basic docs using SuperDuper and a disc image! Well it didn't work last time I tried, maybe it's worth trying again...). That being said, my assessment is that local disaster is not as likely as drive failure and theft, which are also easier to guard against. I would like to have a little bit more piece of mind with an offsite, but that will happen one day. Given that I have e-mails back to the mid-90s you may be able to tell that I'm not one to dispose of any data lightly
