Does transferring files corrupt my MicroSDHC Memory Card??

Here's my case:
1. I attached the MicroSDHC to the card reader coming from Supercard DS One pack
2. As usual, I transferred a few game ROMs to the memory card
3. I realised 2 weird files occurred out of the blue. Both are roughly 3GB. The name seemed to be "USGB??." & "USGB??." (no file extension. I forget the exact name). I tried to delete the weird files in vain.
4. I went to the device manager dialog and stopped the card reader. I removed the card reader when it told me it was safe to remove.

The card was unreadable afterward. My NDSL can't recongize it. All my games and saves, and my personal files are lost.

When I re-accessed the memory card on the computer, it says "The disk is not formatted. Do you want to format it now?"

What's up? Why simply transferring the files will make the card unreadable? Did you experience similar problems?

What can I do now? I would like to get back the files in the memroy card. Obviously the memory card is formatted but it can't be read for some weird reason.

Info:
I bought NDS and the accessories for one month or so only.
Adapter: Supercard DS One
Memory Card: Transcend 4GB MicroSDHC