MicroSD cards stopped working after OSX 10.9 installation, why?
Are they NTFS formatted?  I am having trouble with flash drives that are NTFS formatted.  I use the free NTFS-3G software and am guessing that's the issue.  One post here seems to echo that sentiment:

CNET NTFS-3G user review

Also, found these Apple support posts where people indicate their problem is solved with NTFS-3G and Mavericks

https://discussions.apple.com/message/17356645#17356645

http://osxfuse.github.io/