A post earlier this year by CNET News.com's Stephen Shanklandgot me thinking about the same question.
I can't claim to have come up with "the answer," but I've thought about the issues, read through some discussions about what people consider best practices, and have tried to roughly quantify relative failure rates. What's right for you will depend on priorities and circumstances, but hopefully the following will offer some food for thought.
Real-world failure rates are hard to come by. However, having been the owner of a variety of laptops and other devices with hard disk drives, a 1:100 drive failure rate in a portable device over the course of a month's vacation doesn't seem out of line. Flash memory fails too. Anecdotal information from a couple of dealers (based on product returns) suggests that a 1:1000 rate is a reasonable stake in the ground--10x the reliability of disk. Further complicating the story is that some errors are recoverable, but you'd probably better stop using the card when you have a problem.
That's the hardware. Then there's the wetware--i.e. you.
This one's even harder to quantify. However, speaking for myself, I'm always misplacing loose memory cards. Furthermore, procedures that involve a lot of multi-step copying, editing, and so forth offer lots of potential to erase something that you thought you backed up or for an operation to otherwise fail without your knowledge. Or you might, like me, sometimes just do something really dumb. Also, consider theft and other forms of loss beyond your control.
Add it all up and my guess is that, for most people, minimizing the possibility of human error is more important than incrementally reducing the impact of a potential hardware failure.
With those reliability estimates and human realities as a baseline, here are my thoughts for some reasonable practices:
- If at all economically feasible, carry enough flash memory to hold all your photos. Flash has a good 10x the reliability of hard disks, more when you consider that it's probably going to be OK even if you drop it or run it through the washing machine.
- Common wisdom is that name brands are, in the aggregate, more reliable, and some higher-end cards also come with data recovery software. This seems reasonable. However, I've never seen actual data to bolster this belief--only random stories about crappy off-brand cards purchased on eBay. One data recover company notes that differences in build quality are indeed part of the reliability story but goes on to say it doesn't correlate in any consistent way to brand.
- Because photos can sometimes be recovered from memory cards after they've had a problem, it's a good idea to have at least one backup card. That way, if there's a problem, you can take the card out of the camera and work on it when you get home. Messing with it in the field is a recipe for losing data that could otherwise have been retrieved.
- A lot of people advocate putting fewer eggs in one basket. That is, they suggest using multiple smaller cards rather than one or two larger ones. This is hard to argue against so long as you develop a good system to ensure you don't lose the spare cards or accidentally erase or otherwise mess something up while you're swapping them around. Given overall flash reliability, I don't see this as a particular win--and may even be a net loss if taken to the extreme of some complicated scheme of rotating cards in and out of the camera.
- Although I tend not to bother, making a periodic hard disk backup of your memory cards is good belt-and-suspenders practice. If you're traveling with other people, a hard disk is also a good way to trade pictures. A computer is one possibility. Hard disk-based media players or portable devices specifically designed for the purpose are others.
- If you can't keep everything on flash, then you obviously need to copy it somewhere. Based on the numbers I threw out above, I wouldn't trust a single hard disk backup as my only copy of anything I really cared about. In this case, I'd want either a second hard disk or a way to burn a copy to DVD. (One advantage of making DVDs is that you can potentially mail a copy to yourself at home. (Laptop and DVDs were the solutions that Shankland eventually decided on.) If you have a bunch of spare thumb drives of reasonable capacity laying around, that may be another possibility.
- Cameras break too--maybe more so than any of the other parts we're talking about here, especially if you're in harsh conditions. I'm not sure of the final digital camera mortality rate on the Grand Canyon boating trip I took a couple of years back, but a fair number bit the dust. So definitely consider a backup camera. Sharing memory card format and/or batteries between main and backup is nice, if feasible.
Ultimately, it's all a matter of playing the odds of hardware failure, while keeping in mind all the dumb things that we can do to sabotage ourselves.