I will agree with Tim, save for one caveat: external USB drives are like any technological solution: you need to keep the solution holistic to protect against the future, and you need to evolve your backup mass storage as contemporary technology moves away from the techniques you are using today.
What does that mean?From Part 1 Of a Backup Discussion On My Blog
USB is a wonderful thing -- but it is not guaranteed to be around forever. Even now, the move towards Firewire is gathering steam, and eventually, it is reasonable to say that something even beyond USB will overwhelm USB and eliminate it from the market.
That will not happen today, or even in the next couple of years, but it is a fact that the past is littered with orphaned and obsoleted technologies. Consider 5.25" floppies -- do you ever see a computer sold today with one included? How rare are the drives? They were state of the art in the 1980's, and lots of data was archived upon them then. Now, if you need to access the data, you will have a lot of trouble finding a computer to access the files and import them into a contemporary form.
Further, consider two obsoleted hard drive technologies: MFM and RLL. If you have a RLL drive today, you simply CANNOT mount it in a modern computer. IDE passed them a long time ago. IDE is now evolving towards SATA and I predict in ten years, an IDE drive will be orphaned by whatever has takenm over the marketplace.
Obviously, images are longer lasting than any of that. You will want to see those photos you took at Sentinel Dome in Yosemite twenty years from now. Sentinel Dome will change and you will want to remember the picture you took of the tree that's now gone, for example. I use this because I took a "past and future" photo of the dead carcass of the Jeffrey pine tree that Ansel Adams made in the 60's. It is gone now and the photograph is irreplaceable: you cannot photograph what is not there.
The same thing will happen to USB. That is why there is a need to preserve a holistic solution. For example, I have an old IBM XT computer with a 10 MB harddrive that has one of the first ehternet cards in it. The XT also has two of the aforementioned 5.25 inch drives. I use it from time to time, perhaps twice a year, to export data through the ethernet card and onto CDR for a customer. I charge $10-$20 a disk, depending on if I have to also break out old software that can 'rescue' a damaged 5.25 floppy.
People could and should have spared themselves that cost and trouble by copying their info to 3.5" floppies when they overtook the older 5.25" inch ones in the late 80's and early 90's. Then, they should have moved that data onto CDR when it more or less overwhelmed the 3.25 floppy. Data preservation is an ongoing thing, and expect today's solution to need upkeep through updating tommorow.
And for those of you who do not, I have a Pentium 4 computer already stored away with a stash of software to save your data in ten years.
Gone are the days of preserving images via their negatives in inert sleeves.