The card is probably an MMC as i got one with my 6600. I got the same problem using the card in my external USB2 card reader, however the phone (and a work collegue's card reader) were having no problem reading it. When i spoke to a salesman at The Link he said this was a known issue and due to "copyright protection" the card may not work in all card readers (i have yet to rule out that it could be the MMC slot in the reader that's broken).
Unless you're planning on using it for MP3 and/or video clips, stick with bluetooth and the Nokia suite for file transfers. The only problem with this is that you won't see any hidden folders on the MMC. Check out the forums on the following web sites for info