Transcend 4gb Card & D100: Very Strange Problem

I had used a Transcend 1GB card without a problem for several years, and recently upgraded to a 4GB Transcend. I went out and shot maybe 100 photos, and all seemed fine. I could review the shots in the viewfinder, etc.

When I insert the 4gb card into my reader, for some reason I can only see the DCIM folder. But thats empty, and there is nothing else on the card when I explore it. "Properties" for the card shows about 1gb as used, which should be right. And when i put the card back in the D100, the picts are there and look fine. Both cards are formatted for FAT32.

Any ideas?