I may have a solution
I have the same Issue with a SanDisk 8GB Class 4 micro SD - I was browsing about and found a possible solution. It says shut down your phone, insert card, wait for it to mount so program's can be ran from the card. Then immediately unmount the card and remove it.
Finally replace the card and all should be well.
I am trying it out on a SD heavy program right now and will let you know if it fails.