As already mentioned, external hard drives are preferable to internal drives, since they are easily upgradable in the future by larger-capacity units, and because they allow you to keep multiple identical copies of your image collection, to guard against loss of data due to malfunction or theft.  This is as true for desktops as it is for laptops.  There are a few issues to consider, however, when shopping for an external hard drive.
    The most prominent issue is of course the capacity.  A RAW image file from my 10 megapixel camera is typically at least 12 megabytes (MB) in size.  On a trip to a particularly productive location, such as Florida, I may take 3000 photos in one day, requiring somewhere around 36 gigabytes (GB) of disk space (which easily fits on five 8 GB memory cards).  Over the course of a two-week vacation to such a site, I may fill 500 GB, or one half terabyte of disk space!
    Obviously, most photographers won’t take 3000 photos every day of the year.  Nevertheless, with today’s high-megapixel cameras (many of which are 15 to 20 MP), it’s easy to fill very large hard drives very quickly.  Since there are typically cost savings involved in buying the larger drives (i.e., a 1TB drive may be cheaper than two 500GB drives), I generally buy the largest drives available.  However, it’s important to buy multiple large drives, and to make them mirror images of each other—that is, each of two (or three) back-up drives should contain exactly the same files.  Buying one large drive and keeping multiple copies of files on that one drive is highly ill-advised, since hard drives do in fact fail at higher rates than most people seem to realize.  This is even true of the top-name drive manufacturers: they can (and do) all fail. 
    In addition to capacity, there’s the issue of speed.  When working with large numbers of large image files, it becomes important to be able to transfer data quickly between drives.  Backing up a 1TB drive via a USB connection can literally take an entire day.  There are several components affecting transfer speeds in external hard drives.  The first is the write speed of the drive, which for many drives is 5400 or 7200 RPM’s.  The second is the rate at which data is transferred over the USB or FireWire cable connecting the drive to the computer.  The third is the speed of the controllers and the bus (inside the computer), and the potential for contention among multiple devices using the same bus. 
    Keep in mind that it’s the slowest component that determines the overall transfer rate.  Thus, even if your external drive can write at 7200 RPM’s, if the type of connection (i.e., USB, FireWire, etc.) that you’re using to feed data to that drive is slower than the write speed, then you may be just as well off with a slower (and cheaper) drive.
    In terms of USB versus FireWire, though it’s often advertised that USB has a faster raw transfer rate than FireWire, actual transfer times for large amounts of data may actually be slower over USB, since USB requires the computer’s CPU to direct all the traffic over the connection, whereas with FireWire the transfer of data does not require CPU intervention (i.e., the CPU is free to do other things while data is being transferred).  However, as the competing manufacturers upgrade their protocols (e.g., USB 3.0, FireWire 800), the balance may shift back and forth between them.  At this time, USB is more popular and is probably the protocol that will be around the longest (in some form).

5.4 Options for Backup

While external hard drives may be the most convenient means of data back-up, there are a few other options.  One is to burn files onto CD’s or DVD’s.  This latter option is particularly attractive when traveling, since you can mail the CD/DVD to your home address, and it will then be waiting for you when you return.  Unfortunately, the megapixel race has caused image file sizes to outstrip the growth of optical media.  Single-layer DVD’s, for example, have a capacity of roughly 4.5 GB.  During a recent trip to Florida, I would have had to burn 6 DVD’s each night (on average), which takes quite a bit of time.  Double-layer DVD’s and Blu-Ray disks have higher capacities, but many computers (especially laptops) can’t yet burn these types of disks, and there remains the question of whether the development of higher-capacity optical media (and computers that can write to them) will be able to keep up with the relentless megapixel race in the years ahead.  Note also that optical disks such as CD-R’s and DVD-R’s that are “burned” in your computer aren’t as long-lasting as the CD’s and DVD’s you buy in a music or video store, since the former utilize light-sensitive inks to encode information, whereas the latter utilize aluminum plates impressed with physical indentations to encode their data.
    Another option for backing up files is the use of a remote file server accessed via the internet.  Some of these are even available for free.  Two potential problems with these are the possibility of unauthorized access by other people to your data, and the possibility that the service provider will either accidentally delete your data, or possibly even go out of business with no warning.  The use of these services also depends on your having a fast internet connection, which may or may not be the case for some travel destinations (e.g., deepest, darkest Africa—or even just some cheap hotels in the US).

5.5 Card Readers

Card readers are available very cheaply these days, though not all of them provide the same transfer speeds, and some may not even support newer, high-capacity memory cards (such as 16 or 32 GB).  I keep both a USB card reader and a FireWire reader with me whenever I travel, so that if one breaks, I’ll still be able to transfer files. Furthermore, given that many laptops have only one USB and/or one FireWire port, and given that your external hard drives may use only USB or only FireWire (and you may even have one of each type of hard drive), contention for ports may become an issue when transferring images from a memory card directly to an external hard drive via a laptop.