-Move all apps to phone memory.
-Take the micro-SD out of your phone and plug it into your pc with a card reader and an adapter.
-Back-up the sd card contents so you don't lose anything.
-ERASE the SD card using your computer. Format it as fat32 qith the quick format NOT CHECKED
-Insert into phone
-Format using phone (unmount, format sd.)
-Go through your computer backup and selectively pick what you want on the sd card and move it back. Don't copy everything.
The update error 18 is caused by a faulty or corrupted Market cache file on the sd card.
Using this method worked for me in solving this issue.