Last time I checked with customer care/tech support, they weren't able to tell me why.
Mine did the same thing a few times last year.
I got my CPO Incredible around Feb 2011, set my photos to be saved to the SD card, and a few weeks later I noticed pictures of my kids had been replaced by gray or black boxes. Every time I called customer care, I was told to do a factory restore. The first time it happened, the rep led me through the steps but forgot to remind me to back up my data so my remaining pics were lost I didn't think about it at the time either because I just wanted to know why pictures I'd taken of my children were disappearing. I thought it was an issue with the card so I removed it, but the same thing continued to happen.
After about the 5th time of reporting this problem and restoring my device, I gave up and made sure I uploaded my pics on my computer every week. Customer service wasn't even trying to offer another solution other than restoring my phone.
In November, I got a replacement device from asurion after my original Incredible's screen cracked; I'm used the same 4gb microSD card up until last week when I got a 16gb- so far, I haven't noticed any missing photos. Then again, I don't use the camera on this device much after the issues with my first one.
Message was edited by: HpyNdlGrl
Okay so I spoke too soon... It's 3:15am and my photos ARE disappearing -_-