When I insert my SD card used in my previous Treo600 with pictures on it, to my iPaq 6515 I can't see the SD card by using File Explorer.
Tricks to get your iPAQ to see your SD card.
1) Take the card out and re-insert it. This is quick and easy and works or doesn't.
2) Soft reset the PDA. This will clear any glitches that may have happened. It's also quick and easy.
3) Format the Card on a computer based reader. If the card has been corrupted this may allow the PDA to see the card. You can then format it in the PDA the way you like or just use it.
4) Check the Card on another SD device (formatting it on a reader is one way to do that) to make sure the card is not defective.
5) You can try another brand card. Some brands specs and the iPAQ spec tolerance don't mesh well.
6) Hard Reset the PDA. If your PDA was reading the card fine but quit, this can restore it's ability to read the card. The catch is often this is just temporary and the problems will creep back in forcing another hard reset.
7) Update the ROM. If the PDA has a problem reading a card odds are this won't work, but it does often clear up timing issues and the like. Plus HP will ask you to do this before you send in the PDA.