After upgrading my Mac from OS X 10.6.8 Snow Leopard to OS X 10.7.3 Lion, I discovered I could no longer write files to external NTFS-formatted drives. The problem is MacFUSE which, I found out, does not support the new 64-bit kernel. If this is happening to you, you must remove the old MacFUSE and install the newer version, which does support the 64-bit kernel. Here's how.
Reboot. Once both are installed, you might get an error message like this one.