TJR,
I disagree that portable drives are neccessarily less reliable than internal drives. Hard drives are manufactured by only a handfull of makers and their quality control is pretty good.
Most internal hard drives only have a life expectancy of about 5 years, while an external/portable hard drive can last much longer since it is not always on. Many portable drives are just small cases and connectors that house standard PC or laptop IDE or SATA hard drives, so it is not the quality of the hard drive that causes failures
Complete drive failures are rare but are usually the result of the motor or bearing failures which comes from constantly spinning the disk. In many cases where a drive has errors that prevent the PC from booting up can easily read all the data and files if connected to a PC as a slave drive
Of course a RAID system would be the ideal configuration, but most people do not need that for a home system. Businesses use RAID because they have users that need to always have access to the data and cannot afford to take the entire system down to replace and restor a bad disk.
I have been using external hard drives to back up my PC's for years and have never encounterd any problems.
Currently I use a Toaster style external drive unit that allows me to back up my desktop, Laptop, Netbook and my office laptop on a a single 1-Terabyte hard drive. I schedule my desktop back ups weekly with the standard Microsoft Back up that comes with Windows-XP. I have been doing that for over 2 years and it has not failed me yet. If I something important on my computer like my taxes, etc., I will force an immediate backup of that directory just incase.
...Rich