Then using our largest reflector the LR24 (it has the longest magnet), we placed the cards 3 inches from our magnet for 20 minutes, then 1 inch for 20 minutes, then the cards touching the magnet for 20 minutes. After those tests we rubbed the two back and forth on each other for 2 minutes. Next, we then sandwiched the cards between the two LR24 magnets for 20 minutes (if that didn't damage the files it never will). Between each session we downloaded the photo files.
We encountered no problems mounting the cards or any difference in images from the control to any test sessions. In fact, all these tests were with the SAME cards, no compounding effect either.
See sequence of photos below. We also laid a floppy disk on the same magnet for two hours, no interference detected. Our magnets are OK with your media.