Why would my computer open a SD card one day an not open it after that?

Hi, rhoni!

Had the card been altered in any way before making the attempt the second time after opening it the first time?

Thanks!

In addition to Clough's comment, have you formatted the card for a certain computer? I know that during the holiday, I wanted to download my pics from my Nikon card to my brother's computer, but it didn't work because it was formatted to a PC - and he has an Apple...

Could be a possibility?

Or, did you lock the card? There's a little switch on the card itself - check to see if it is in the locked position.

That's an excellent point about the locked position, HistorianChick!