SD Cards "forgets" written data
It sounds like that SD card has a faulty controller chip. From your description, I'm guessing that you bought it second-hand through ebay, so you might have been "burned" there.

Something you can try is to open Disk Utility, choose the SD card, and then click Verify Disk.

Something else you can try (crossing fingers) is to reformat the SD card and hope for the best. However, if the SD card is acting flakey, then it might be best to scrap it for a new one.

Question: In Disk Utility, select the SD card and choose Info. What File System is the card formatted for?