Nobody knows why and nobody can predict when, but occasionally someone reports that the memory card got corrupted. Meanwhile hardly anyone has reported a corruption problem (other than a complete failure of the card) when the card was formatted not too long ago.
Formatting is inconvenient for those of you who don't offload everything to the computer at once or for those of you who want to keep a few of the best pictures to show to friends frequently. This in turn results in the card's going for a long time without being formatted again.
I reformat in camera after I have copied my pics over to both my EHD's. So, I reformat after just about every time I use my camera since I load my pics to the computer, EHD's usually the same day I took them.
I've had mine for about 3 yrs I guess & have never reformatted but my MIL was sure to tell me that I needed to regulary so I wanted to see what the DIS boards said - y'all know way more than her! LOL
I also reformat my cards via the camera each and every time I pull them out to transfer the images to the computer. It is a quick process and makes me feel a bit reassurance that he cards will stay in shape longer.