Can you tell me how to check MD5 sums? Is there a utility for this?
Yes, there are quite a few. If you use Windows, try one of these:
http://www.google.com/search?q=windows+md5sum
What you need to do is select a set of files that just about fills up your
card, compute the checksums, and copy the files to the card. Then eject the
card, (perform any steps dictated by your card reader and Windows software
interactions to purge any cached card data; rebooting at this point should be
nearly 100% effective), reinsert the card, and verify the checksums on the
files on the card. If they match, you're probably ok to use this particular
card in your damaged reader.