There are some counterfeit Sandisk cards and your post is not uncommon.
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I recently bought an 8GB Sandisk SD Card from Ebay. I had decided to use it for storing MP3 files so I could listen to them on my new GPS device. However, when I try to uphold more than 2GB of songs, some of the songs are randomly deleted. When I try to re-add the songs, roughly only 10% of the songs stay transferred after I safely remove the card from the computer. I've tried reformatting the card to a different file system like ex-FAT but the problem still occurs.
I'm using a Compaq with Windows Vista Ultimate. Thanks in advance for your help.